I would make both the merchant and the bandits/assassins in the final level non-switchable, so you cant move them around before the fight.
for the bandits/assassins make them so they are immovable or say something to that extent when you bump into them.
for the merchant do this (or something):
Then go into the quest tree for the assassin lord.The merchant begs you to help him, only to be cut off angrily by the assassin lord, who silences him with a blow to the head. He staring at you, he states, "This prize is mine, begone stranger!"