All potential bonuses

Sorry if this has been asked before: does total damage and boss damage stack?

Also, say you get 30% damage on one weapon and 20% on another, or 30% and 20% on the same weapon, how does the game determine which one takes priority, since boss damages don't stack?

I can only answer the second question, Cyanne got her 30% boss dmg on a Dagger, and 20% on Katara. The end result was +20%.