Huh, had more time to play around with the corruption mechanic, you can actually prevent HORNET from going evil for as long as you like.
When queen hornet gets fully corrupted (happens over time, first time on day 15), she will attempt to convince the HORNET squad to go rogue and fight NASCA, if all the girls are already at maximum corruption they will agree and you can't prevent the game from moving to the next stage, however, if at least 1 of them isn't fully corrupted, a fight will break out between the corrupted girls and the ones still holding on. For some reason, you play the fight as queen hornet and the corrupted girls, so if you want to prevent HORNET from going evil, you just have to lose the fight on purpose by skipping your turns and not beating your adversaries. After losing the fight all the girls will lose 10 corruption.
So the only requirement to keep going forever without becoming evil is to make sure at least 1 girl isn't fully corrupted. Red and Blue have events to reduce corruption if they patrol the park, but they are random and can fail, increasing your corruption instead. Yellow on the other hand, it seems that after her first visit to the Residential District (the first time she might gain corruption if she is already a bit corrupted), every visit then seems to reduce her corruption by 5. So if you wanna keep going forever, you just have to keep Yellow patrolling the Residential District every turn. Also Blue sometimes gains reputation when patrolling places that aren't the Park, so you can max out your reputation to gain more money from NASCA, EDIT:I read it wrong, Blue loses reputation, Yellow is the only one that can gain reputation (I think Red also has an event that gains it but only at low corruption).
EDIT: Yellow will not lose corruption in the residential area event if she is already fully corrupted, so you have to be careful that she doesn't gain too much corruption during the fight against Queen Hornet (try to run away from her as much as possible).