When you are with parties you aren't supposed to be able to play the monster in matchmaking. However it seems this is broken when playing with multiple parties in lobby, its happened to me multiple times since release.
Otherwise roles are assigned by the preferences you set but you can back out a level and switch with people if you want.
Ya I've been having trouble with the controls as well. I've played about 3 hours now and things still aren't really coming together for me so I need to try using my 360 controller. Also it makes no sense why you can't bind to mouse buttons, it seems like such a silly thing to prohibit.
I just don't see this slowing down when these assholes keep getting real world results with these threats. It's really fucking sad to see people treat each other like this and I honestly don't know what we can do to make this kind of behavior end.