After Tom's downfall, the main action against his supporters was by the Ministry. (BTW the rumors must have started flying before morning. Who started them? Bathilda Bagshot? The Ministry clean-up team? Hagrid? Albus himself?)
Alastor Moody and Frank Longbottom still had quite a few names on the list provided to them by Severus, by way of Albus, as well as at least one name (Nott) from Albus' original list of the very first DEs. There were (probably) also people who were captured based on crime-scene evidence and eyewitness testimony. Some were eventually put to trial. Some were sent to Azkaban without trial. And some were let off without trial. Why the difference in fate of the various accused people? One thing to remember is that Crouch was a dictator and the hallmark of dictators is that they enjoy displaying power - and that includes both the power to punish and the power to arbitrarily show mercy. Was there some method as to who ended up in which fate? I don't know for certain, but I notice the following: The people who were jailed include Sirius - estranged son of prominent family, suspected with contributing to the deaths of the Potters; Dolohov, accused of taking part in the killing of the Prewett brothers (purebloods, Order members); Travers - the only known DE member of an old pureblood family to be incarcerated before the attack on the Longbottoms, accused of involvement in the killing of the McKinnons (one of whom was an Order member). The people who were cleared include 3 members of old pureblood families with direct or family ties to the Slug Club (Lucius Malfoy, Nott and Avery). The Lestranges are an interesting case: members of old pureblood families, managed to be released the first time around, then got caught for their torture of the Longbottoms (purebloods, Order members, at least one of whom was a 'popular' Auror) and were incarcerated. It almost looks like whether one was incarcerated or cleared depended on a calculus that considered the relative social and political standing of the accused and the victim: One's chances at getting cleared looked better if one came from a high-status, well connected family and the victim did not. (I wish we had information on non-DEs besides Sirius who were suspected of DE crimes, and how their fates turned out. Well, hwyla and Terri proposed that Aberforth was one such person, though the accusations against him ended up being low-level enough.)
Another thing that happened around that time was that many suspects claimed they just realized they had committed the crimes with which they were accused while under the Imperius Curse. The Ministry had a hard time deciding which of these claims were genuine. We do not know how many of those who were truly under Imperius remained incarcerated and how many were released, nor do we know if any of the DEs who falsely claimed Imperius were caught in their lie. We do know that among the DEs that were released were Nott, who was known to Albus as a DE for decades, and Lucius and Avery, who must have been known to Severus as DEs. How were claims of Imperius investigated and how did some DEs get off? The consensus of the best minds of this board is that one might be able to use a combination of Veritaserum and Legilimency to investigate the claim, while taking precautions for antidotes to Veritaserum. But I doubt they investigated all those they should have. For once, anyone who was arrested for crimes committed while under Imperius before Voldemort's downfall (and may have even confessed for those crimes) was being preyed on by dementors. These people lost the capacity to advocate for themselves, and probably lost the ability to recognize that they had not acted on their own will in commission of those crimes. No, the investigation was limited to those who came forward after Halloween, as well as possibly people who got arrested - if the Auror came to suspect this was yet another case of Imperius, or if the prisoner made the plea after arrest on their own.
How did Lucius and others convince Crouch's DMLE to accept their pleas when at least some of them must have been on the lists of suspects that Albus passed on?
We don't have much canon discussing this matter.
In PS chapter 4 Hagrid says: "Some say he's still out there, bidin' his time, like, but I don't believe it. People who was on his side came back ter ours. Some of 'em came outta kinda trances. Don' reckon they could've done if he was coming back." (BTW a bit surprising from someone close to Albus. Hasn't he been insisting for years that Voldemort was bound to return some day?)
In PS chapter 6: "I've heard of his family," said Ron darkly. "They were some of the first to come back to our side after You-Know-Who disappeared. Said they'd been bewitched. My dad doesn't believe it. He says Malfoy's father didn't need an excuse to go over to the Dark Side."
From these two quotes it seems Lucius didn't wait to be apprehended for interrogation, he came forward and confessed. However, just one person claiming to have been under Imperius would look strange. His plea would be more believable if quite a few people made similar ones. And even better if most of them were not also associated with known or suspected DEs. So I think it was not by chance that many true Imperius victims came from under the spell's influence at the same time that Lucius made his own plea. Those were, for the most part, his own victims, or perhaps victims of fellow DEs who were going to make the same plea at the same time (more or less) - the casters released their victims to create a trend of peopl coming out of the spell's influence so they could hide among them. In OOTP Harry suspects that Lucius Imperioed Sturgis Podmore and Broderick Bode. In HBP beginner-DE Draco manages to keep Rosmerta under Imperius from October to June. Maybe Malfoys have a talent for this curse. If so, we have an explanation how a DE low in blood-thirst such as Lucius managed to rise to a very high position, as the graveyard scene in GOF seems to indicate.
Five (or maybe 8, if we the Lestranges also used this route initially) other DEs were cleared with Lucius. Why them? Why not others? Some of them may have been members of Lucius' terror-cell, his partners in crime. Others - if he was already high ranking, he may have known more DEs beyond his immediate cell. Or if there were DEs who were close friends of his from earlier times, perhaps from school, and he guessed their identities in the circle - he could have joined forces with such friends to make sure neither of them stayed imprisoned. Do note that 3 of the 5 had sons in Draco's year. It is possible that Lucius sought out those DEs who had young sons to match their defenses and have those sons loyal to Draco. Alternately, the other fathers may have sought Lucius out, to model their defenses on his own. Or a variant on the above.
How did those 6 or 9 DEs talk their way out of Azkaban successfully? Considering that Crouch's regime used Unforgivables against suspects I doubt they had trouble using additional means such as Veritaserum or Legilimency (the latter being limited only by access to a sufficiently trained Legilimens).
Which means anyone trying to avoid incarceration would prepare for such eventualities. One can perhaps avoid consuming Veritaserum like Harry does in OOTP, though I expect supervision by Aurors to be tighter than Dolores' supervision of Harry (also, it should be easy enough to Imperius the suspect to take the potion if deemed necessary). One can perhaps attempt to get an antidote smuggled in - though any antidote should only help for a limited time. Both of the above work better with inside help. Perhaps by bribing or Imperiurizing Aurors. Or Crouch himself. Or by having some DEs working legitimately in the Aurors' Office. After all, there were at least 24 DEs in the circle whom Tom did not mention by name. (Though had some DEs stayed under the radar while holding positions of Aurors wouldn't have Tom called them out in person for not seeking him?)
Safer yet is to make sure one does not have anything to confess beyond what one claims to have been forced to do by Imperius. Have a trusted collaborator who was unlikely to come under investigation themselves modify one's memory to match what one is planning on confessing. Have them restore the original memory when things were safe again. While we do see Albus and Tom see through modified memory, it isn't obvious any run-of-the-mill Legilimens can do so. The fact that these DEs, (including Nott, whom Albus knew as a DE ever since Tom returned to Britain) slipped through is evidence that Albus was not involved directly in their interrogation.
Of the sentenced DEs only one talked his way out, in a different fashion. Igor Karkaroff turned Ministry witness against his colleagues.
Before naming any DEs he says, "And these were important supporters, mark you. People I saw with my own eyes doing his bidding..."
Igor reports the violent acts of 3 DEs: Antonin Dolohov - torture, Mulciber - magical coercion, Travers - was one of the killers of the McKinnons. He probably would have mentioned the deeds of Evan Rosier too, had Crouch not stopped him with the news that Rosier was dead. These 4 are the only ones Igor saw with his own eyes in action, the members of the same terror cell. If there ever were any others, they were already dead or imprisoned by the time of Igor's capture (no earlier than mid-July 1981, because he was present when the McKinnons were killed). So maybe Wilkes was also in this cell until his death.
I find it odd that a terror cell that included one of the original DEs (Dolohov) performed its only known murder as late as July 1981. Dolohov isn't wand-shy when we see him in action in OOTP or DH. Perhaps Igor didn't want to implicate himself in more murders than he was already charged with for fear that Crouch might decide to keep him imprisoned no matter how many of his colleagues he hands in.
Once the aforementioned 4 names turn out to be useless, Igor names two additional DEs, for whom he has no acts of violence to report. These are the spy Augustus Rookwood and one Severus Snape. If they did not participate in DE attacks, how did Igor knew they were DEs in the first place?
From his description I think Igor must have been significantly older than Severus, I don't think they could have known each other from school, even if we assume Igor was raised in Britain despite his foreign name (in the book he speaks with no noticeable accent, in contrast with the movie). But if Severus was the DE healer Igor may have seen him in this capacity. And he may have recognized the healer as the masked man who stood next to him in the circle, whether by voice or gestures and body language.
I am stumped about Rookwood. Were they friends from before they became DEs and dropped cover for one another? Did Rookwood need assistance from Igor regarding his spying? I think Rookwood was among the older DEs because he was friends with Ludo Bagman's father. Also, isn't it strange that Igor never gave Rookwood's name away during his pretrial interrogation, considering that the Ministry in those days let Aurors use torture and mental coercion freely? Or does this mean such methods actually aren't so effective at revealing secret information?
What can we learn from the fact that even when desperate to give any name he could think of Igor doesn't give away Lucius Malfoy's name, nor the name of any of Lucius' fellow Imperius pleaders or the name of Bellatrix Lestrange? This suggests that he couldn't recognize them as any of the masked and hooded people in the circle. Making Igor even less observant than Harry, who was able to recognize the masked and hooded Lucius at the Ministry in OOTP after seeing and hearing him on a handful of occasions. (And how many female DEs with a tendency to speak in babytalk can there be?) Unless Igor had even fewer opportunities than Harry to meet Lucius (and Bella) unmasked? This supports Igor being foreign-raised, or if British then old enough to have left Hogwarts before Bellatrix started though young enough to have missed Nott and other early followers of Tom. Thus if Igor attended Hogwarts he was likely born no earlier than 1934 and no later than 1945.
According to Crouch the arrest of Antonin Dolohov and the death of Evan Rosier (at Moody's wand) took place shortly after Igor's arrest. In OOTP we learn (from the Daily Prophet) that Dolohov was arrested for his part in the deaths of the Prewett brothers. Earlier Moody tells Harry that Gideon and Fabian Prewett were killed by five DEs. Well, after Igor's arrest his cell consisted of Dolohov, Rosier, Travers and Mulciber. Who was the fifth attacker? One possibility is that a new recruit was attached to this cell. Another is that another cell lost several members (killed or arrested) and the remainder of its members were split among more intact cells. For those in favor of more twisted scenarios - this would be the ideal use of Imperiurized!Arthur - forced to kill his brothers-in-law. (Members of the Weasley family were within hearing distance when Moody showed Harry the photo, so why would Moody bring up such a horrible story even if true?.) The timeline suggested by Crouch (in combination with Moody's comments to Harry in OOTP and Lily's letter in DH) places the death of the Prewetts in August 1981, perhaps very close to Ginny's birth on August 11th 1981. Did the news of her brothers' deaths start Molly's labor?
To Igor's disappointment Albus Dumbledore spoke up and mentioned that Severus Snape was already cleared by the very council that was hearing Igor's attempted plea bargain. Somehow there were never rumors about Severus' past, at least not at Hogwarts. Ron and Percy passed on some tidbits about Severus' habits and interests, but there was no mention of rumors about investigation for DE activity (as opposed to the situation with Lucius). The implication is that Albus managed to keep the students from noticing that an investigation against a teacher was going on. Perhaps by having all the interrogations take place out of hours, and on the weekend(s), concluding the investigation during Christmas break at the latest. With Lucius and his friends coming forward and not knowing who else might speak Albus did not want to be in the position of having to explain why one of his employees was being implicated by people the Ministry was interrogating.
How was Severus' interrogation handled? We don't know, but canon shows that Albus never cared about him as a person, only as his tool. And in GOF we see Severus avoiding 'Moody''s eyes. Considering how Albus treats Hagrid, Remus or Aberforth - I'd say Severus was thrown to the Aurors with a note from Albus saying 'do your worst'. After all, intervening on behalf of a new hire who used to keep bad company would be suspicious. What better way for Severus to clear himself than being subject to the same treatment as all those, innocent or guilty alike, suspected of DE activity received? And after all, they both knew Severus wasn't an innocent in the first place. Between mentioning the help his spying did to the war effort and arranging for his interrogation to not interfere (too much) with his teaching, Albus did what he could to help. And so 21 year-old Severus spent a few months in his first year as a teacher alternating between the Potions classroom and a Ministry torture chamber. Possibly with a couple of dementors for company during that time, as Umbridge's system works in DH.