Merc's-Single Player

I'm up to LvL40 in single player. Woot. Anyways, I've had a merc w/ me the whole way and it's basically like having another player, we own everything. Well he was a concentration merc, which was nice cause I had atleast 75% chance to hit, pretty much everything. The minute that I entered the Worldstone Chamber to woop Baal, he morphed into a conc/might merc... Is this normal? I've had merc's online, but I always lvl them up from nightmare, never all the way.
Also it seems that his Might adds to my damage, along w/ my fantacism. Does it damage stack? Say his might does 50% ED, and my fantacism does 75%, will I be getting 125% or just 50%, and then 25% more after that? Does that make sense?