A) The MC rolls, yes.
B) If the defending force chooses to rally and make a counterattack, as a defense option, then the attacking force becomes the defender. This is the only way.
If the attacking force decides not to press the attack, but to hold ground or retreat instead, then this battle ends. Everybody should count their fallen, and you should find out what everybody does next. If, at this point, the defenders launch an attack, treat it as a new battle.
-Vincent