1&2) Afraid I can't answer for end game dungeons. I've done up to level 58 with 5 clerics (and I really wish it had been 4 clerics and a mage). Levelling dungeons are harder than wow heroics except some cata heroics, I wouldn't be eager to try and raid or do experts. The five mans are definitely a challenge and I appreciated it at the time but I'm going back to wow, rift just doesn't engage me long term.