waterbolt is being buffed by 9% and is better to have than not have, plus there is still time for mastery to change again before patch hits live, not like i care since i quit the game ages ago, i just keep checking back every now and then, knowing blizz they will over buff it for when it goes live, then super nerf it a week after the patch stating it needed "balancing" or some other BS like they have done this entire expansion for mages soo far.
Icicles shoot themself on their own if you get more than a stack when you're already at 5 stacks. So unless you wan't to use all 5 icicles for burst over like 5 seconds (because what I've heard the icicles are extremely slow), then be my guest.
Besides, according to venruki (and what i've seen) icicles do literate shit damage in PvP because of double dipping resilience.
Ice Lance is the only spell that will activate all 5 icicles, the other spells just use up the oldest one in the stack, so you will always be able to choose the target that will get all 5. The stacked damage will not be easy to control because of the dynamics of pvp.
As for haste or mastery, we won't know this until a week before the patch anyway, the numbers will be changing often.
12k of that is stored in an icicle. You can have 5 Icicles. Thats a total of 60k Damage. 60k is 60k, nothing special. But afaik Frostbolts don't hit for 100k in PvP so 60k is more of a PvE number. So 60k in PvE numbers could be more like 20-30k in PvP. That's nothing.
Sorry to break this to you but Icicles is a way for PvE to keep their damage up after the nerf to PvP.
Also in PvE turning your welly to passive will be a DPS loss. Welly bolt in PvE doesn't lower your Icicle damage. It is the opposite infact.