Hmm im not sure what your problem is.
I have 28,5k mana ub (im still using the int trinket since i dont have anything better)
I do aswell run out of mana by end of P1 with 1 managem gone, however I do fish for my procs during the whole phase, some tries I had to stop fishing at around 75% boss HP due to mana probs, but the moment P2 starts I use evocate and no more probs after that.
Also I cant see why arcane sholuldnt be able to do decent DPS in this fight. Okay, we always have a disc priest which gives me quite a lot of absorbed SP over the whole fight, by the end of P1 10,5-11k DPS is always possible... of course it drops after P1 but still...