The only new name of a DE Igor was able to provide was that of Augustus Rookwood, the spy within the Ministry. Yet this information was sufficient for Crouch to release Igor, an admitted participant of the raid in which the McKinnons died.
The only way I can see this working is if the capture of Rookwood's spying network was seen as more important than all the lives taken by Igor and friends. Why was that so important after the war? These people weren't DEs, they weren't killers, they merely passed on information, sometimes (often? almost always?) without understanding how their actions were aiding the enemy. Thus finding out about them meant Crouch knew who in the Ministry was weak, who could be tempted, confused or manipulated. Or blackmailed. Very useful for someone who was planning on becoming the next Minister for Magic. If Sirius is correct in the information he gathered well after the fact (while on the run) some of them did end up in Azkaban, for a while. But those who did not (and of course those prisoners that completed serving their respective sentences) - Crouch knew about the skeletons in their respective closets. (There is a bit of a problem with Sirius' narrative - why would Igor fear going back to Voldemort more than other DEs after putting several dupes in Azkaban? Yes, he did lead to Rookwood's arrest, but then Severus caused Tom worse problems and survived for as long as Tom didn't think killing Severus was necessary. Couldn't Igor have claimed he was busy corrupting eastern Europe's youth for Voldemort's sake? There he was, teaching Dark Arts to a grandson of Greendelwald's victim.)
At some point the investigation got to Ludo Bagman. We did not see his entire trial in the Pensieve. By the time the scene we saw unfolded the evidence stage of his trial had concluded, the session we saw was the sentencing, after he was already found guilty of 'passing information to Voldemort's supporters' and already gave his testimony regarding the events. In his sentencing session he was given a chance to speak once more. He did not deny his actions, he only denied understanding that the recipient of his information was a DE, and provided a motive - expecting a favor in return (a rather significant one for young Ludo). Which means the previous session(s) were only about his actions: maybe Rookwood testified, or perhaps the Auror(s) who had interrogated him. There may have been an eye witness to their meeting. But until Ludo brought it up at the sentencing, there was no discussion of his motivation or his understanding of his actions at the time. Why was that? Did Crouch prevent Bagman from explaining himself in previous sessions? ('We are not interested in what you were thinking, Mr Bagman, stick to the facts. Did you or didn't you bring those documents to Rookwood?') Or was this Ludo's attempt at a defense something he came up with once he realized he was going to lose on the facts and evidence front?
Crouch proposed a sentence of imprisonment lasting no less than - and got cut off by the jury. Meaning even Crouch had no intention of imprisoning Bagman for life. And note that Bagman was not accompanied by dementors, nor was he bound to the chair. Crouch did not consider him a flight risk, though he did believe Bagman should be imprisoned for a while.
Elkins pointed out the following: While in Igor's hearing Crouch is described as 'fit and alert', by the time of Ludo's sentencing he is described as tired, gaunt. And the public rebels against him - cutting him off mid-sentence, crying out, shaking their fists. Elkins connects these items - soon after Godric's Hollow the public was still in wartime mode, people wanted Crouch's tough policies to keep them safe. But a few weeks or months later, as they adjusted to the peace, they no longer wanted to hear that their friends, cousins, or even sons may have been on the side of the villains. Not even as dupes. The threat of losing friends and family to Azkaban was worse than the threat of war, the government worse than the official terrorists. IOW Sirius was wrong in his assessment that the turning point of Crouch's political career was his son's trial. It was in the lead-up to Bagman's trial. I propose that shortly after Igor's hearing Crouch was beginning to feel the change. Other than Rookwood there were hardly any DEs to put to trial. The evidence against remaining suspects was shaky, many were pleading Imperius and it was becoming hard to counter their pleas (well, these people were actually honest, the liars were already cleared early on). Bagman's case looked like a gift of the fates - someone who did not deny the facts, nor did he claim to not being in control of his actions. Crouch was hoping to make an example of him (capitalizing on Ludo's celebrity status to enhance the lesson). Because Crouch's quest for catching one more DE in order to restore his lost popularity was already underway.
He should have been careful with that wish, for it was granted soon enough. When the suspects in the torture of the Longbottoms were caught. And his son among them.
The Longbottom affair is one of those canon stories that generated many conspiracy theories. In part because of the timing - showing up in GOF, before the long wait for OOTP, but also because so little information is actually given in canon about it, while being an important piece of Neville's backstory. None of the conspiracy theories I have encountered so far received confirmation in DH. I no longer believe any of them, but now I have a shiny conspiracy theory of my very own.
What do we know? The three Lestranges, with the possible assistance of Barty Jr tortured Frank and Alice Longbottom for information about Voldemort's whereabouts, causing their victims to suffer permanent insanity. The four of them were caught and put to trial. However even years later Albus Dumbledore expressed doubt about Barty's guilt. We meet Alice Longbottom in hospital and see her handing a gum wrapper as a gift to her son, confirming that indeed she isn't all that connected to reality.
OK, what was the evidence that was presented in the trial?
Albus says, "... Unfortunately, the Longbottoms' evidence was - given their condition - none too reliable."
IOW the foursome were convicted based on eyewitness testimony of their victims. Three of the perpetrators are hardened terrorists. They were not caught in the act. But they somehow did nothing to silence the victims? Why were the Longbottoms let to live at all? OK, maybe one of the four (or three) noticed the floo acting up or some other sign that someone was coming, warned the others and they all left in haste.
This is what Crouch says at the sentencing:
"The four of you stand accused of capturing an Auror - Frank Longbottom - and subjecting him to the Cruciatus Curse, believing him to have knowledge of the present whereabouts of your exiled master, He-Who-Must-Not-Be-Named . . .You are further accused...of using the Cruciatus Curse on Frank Longbottom's wife, when he would not give you information..."
Elkins pointed out that the word 'captured' implies the torture did not happen at the Longbottom residence. They abducted Frank, brought him to the place of their choosing (the Lestrange estate dungeon?), tortured him. When that was not working they went and abducted Alice. With the perpetrators having control of the time and place, I don't find the scenario of them being surprised and needing to leave quickly very convincing.
Based on Bella's attitude in OOTP and HBP I think she sincerely believed her effort was likely to result in the information she wanted. Her choice to attack Frank was based primarily on a rationally held belief that he might have the answer, suggesting to me that he was involved in the search for Voldemort, maybe even as head of the team. Other considerations such as a vendetta against one of Crouch's Aurors or a family conflict played secondary role at most. Frank's position would be higher than that of Kingsley Shacklebolt as head of the search for Sirius Black from June 1994 to June 1996. (Unless by then the search for Voldemort was no longer of high priority to the Ministry? Perhaps the Ministry decided that if Voldemort has not returned within a few months the best policy was to let disembodied wizards lie?)
So at first the Lestranges tortured Frank (for how long? hours? days?). When they brought Alice he was still coherent enough, as their purpose wasn't to get the information from her, the goal was to use her pain to break him. From his reaction they must have realized he really did not know. So what now? Sure, they could kill their captives, but how will anyone find their master for them? I propose that at this point the Lestranges decided to Obliviate the two, in order to cover their tracks, and to release them, in hope that at a later time the same series of actions would turn out productive. But the gaps in Frank's memory raised suspicion, and someone tried to find out what was missing. It was this effort to retrieve the lost memory that finally destroyed his and Alice's minds.
(BTW that Albus doubts the evidence the two provided means they weren't given Veritaserum either. Supporting Terri's hypothesis that the potion was not yet known in 1981-2.)