Neil will not be affected because, trust me, just because the warden will be forgiving of some rules broken during the event, there are certain rules everyone can appreciate, and they all involve the use of firearms. I'd rather not have him locked up yet.
--though perhaps something can be worked out if someone can keep him sufficiently distracted to where thoughts of murder are far away. Or get him drunk enough. Something!
Touma might be affected, but I'm still trying to figure out exactly how. It's hard to brain when all his inhibitions are to keep one specific secret hidden; one that he works very hard to keep and does so for not entirely selfish reasons. If anything he'll just be more open and honest, more easily provoked, and prone to swings in his mood he loses the patience to constantly monitor his reactions.
Subaru is already the most open out of all of the above, but she will just be more so. Basically, this event will undo a lot of the work she has put into thinking before she acts, doing and saying things without first considering the consequences. Sometimes these are good things -- expect affectionate greetings if you are her friend. Certain people are liable to earn an "I like you!" at some point.
Those who cross her or threaten others might be in for some serious shounen speeches. She still won't fight someone unless she's left with no other choice, but when she does, it might be harder for her to hold back and not seriously hurt someone (or come close to it).