Man, you guys need to chill about brainwashing. Can you come up with a reason for why any of the stuff that happens in the games/shows happen that makes some sort of sense? Seriously, there is so much that has to happen in such a short amount of time that no matter what the reason was, it was going to either feel weak or unrealistic.
Each member of the DR2 cast had built-in reasons for why they would want to see the world go to shit in the game they originated from. Combine that with Junko's perceived charisma before DR3, and it'd just be a simple means of "come with me to the dark side" bad guy speech like she did for Izuru, but for everyone else as their life situation devolves (Twilight Syndrome would be a crucial part of this), thus amassing her cult of personality. It's not the most inspired idea, but it's at least the expected one and it would work just fine within the context of the DR universe. It seemed to be implied that the world burned not because the brainwashing video was broadcast all over the place, but specifically because of Hope's Peak imploding. DR2 cast's heel turn + first killing game + Reserve student madness was all that was needed, and it could have been achieved in any number of ways.
Brainwashing for the mass suicides and the craziness of the Reserve students? Sure, yeah, whatever. They're not the crux of the story; use that Diabolus ex Machina as a crutch to move the story forward. But if it was always going to feel weak or unrealistic to have a 12-episode prequel anime, maybe it shouldn't have been done in the first place?