Haukke HM:
1. 3 dead skeletons inside the big purple circle. Watch them, they'll all be pulled to the centre, connected by a purple line. Then they'll merge to form a big skeleton.
2. Last boss: I feel that way too. Usually if DPS is too high, adds will die quite quickly, and you might get back to back Blood Rains for the 2nd part.

Copperbell HM:
1. Running too far from the boss prompts him to set parts of the bridge on fire. You can kinda "speed up" the rate he burns the bridge, although I don't know why you would want to.
2. Yes, DPS should handle the fire sprites, and tank should carry bomb. However, seems another way to do it is 1 DPS carries the bomb, and the other picks up adds. Gogmagolem's attacks are kinda random, sometimes he does Skull Bash, sometimes he does Landslide. At the 3rd phase I notice he doesn't use anything other than Earthen Heart.
3. Never tanked the adds before. Usually the melee goes to pick up the adds, and the ranged does the rocks.