Okay.
Right after Crossover AU is done (or I might pull Sol out early, because this next part is crucial as anything, considering it explains what the fuck is actually going on in Overture and what Valentine's after), post the next cutscene mission - 12 - because it explains what the hell is going on better than I can write it right now. Skip the Sin-centric mission for obvious reasons - he's run out by then.
Same with Mission 14 - it's from Izuna's perspective.
After a few days from 12's posting, cut to Mission 15 - Sol beating up brainwashed!Sin.
Mission16 is skipped, mostly because it's Sin-centric and otherwise pointless for Sol. :|
MISSION 17. THE BIG 'UN. SOL VS. THAT MAN. WITH SOL IN GEAR FORM. FUCK YEAH. Definitely writing this one out.
Mission 18 - skip. Sin's perspective. Sol's too busy trying to reconfigure the Cube to be answering anyway.
Mission 19 - Show the beginning - where Valentine shifts from her first form to the other one, cut out as Sol goes in to fight it alone. Most of this will be off-camera because of the sheer annoyance it would be to write out fully, plus Sol's had enough trauma from being consistently reminded of Aria thanks to Valentine. Probably for the best to cut him a break right now. D:
That should take care of most of the actual game - Mission 20 is another "cutscene mission" and the conclusion of the game, and it's all over the place. Watch the cutscene to try and make sense of it - Sol goes with Izuna to try and make more sense of what happened, then he'll probably head back to Illyria to pick up Sin - eventually. :|