I didn't read all of the post's here but... i think it would be easier if you just add a world customisation option to make enemy's harder to kill or make them deal more dmg to the player, the creator of the room should place this options and the ppl then decide if play that world or just try to find another one with less dificulty. when ure looking for a room to play, the game should show you the option's that the owner selected. It's gonna be for the best for veteran player or new player's. And for the friendly fire... you can make it only happen if the player have become really insane, in the game of course lol (?)