Ok i didnt read any posts but the first one but heres your solution. Since its pretty obvious you and your cousin are in there for the game and not to win, which alot fo the crappy L4D players do. Lose the first 1-2 rounds, like badly, pretend u dont kno what the hell you're doing, maybe even say that in chat, like laugh alot. Then rounds 3-5 own face. Me and my roomates do this and it works pretty well. There will always be rage quitting because people are lazy and dont want to face tough teams, and the decent players that get stuck on a team that half of them quit from leave because 4 v 2 is just no fun at all. The real solution is some sort of penalty against people quitting before the game is over. Nothing crazy, maybe a 5 minute penalty where you cant play versus. Valve did there best to fix it by making it that the losing team gets to go infected first but I think a time penalty would work well.
edit: and forget the feedback system, that never works
edit: and forget the feedback system, that never works