My biggest concern for your formation is the swapping of mercs entirely for different scenario's vice just formation.
I'm sure you are feeling the same pain as I am right now with the leveling process after 95. I think the formations themselves are damn fine... Will you be happy though with the swappage and the leveling process of other mercs between pvp/pve?
My only thing about the witch... I actually enjoy her for pvp more... having her in my center really jacks up art/can merc/mains. Other then that, I really hate her for pve.