1. If a hero is in play and ignored, it can't enter battle.
Characters that are in battle and ignored are treated as if they were not in battle for purposes of determining battle outcome, so you shouldn't need to block.
2. Outside of battle, I'm pretty sure you can choose any player to be your opponent. During battle, if you are involved in the battle, your opponent is the player on the other side of the battle. If you're not involved in the battle, your opponent is any player at the table not involved in battle, or no one in a 3 player game.
