Tesla Cage is a hold, not an Immob. Handy in PvP when combined with Pet' Gaze to stack holds on a boss.
Voltaic Sentinel does something like 10x Charged Bolts. Might be useful for firing whilst mezzed, I guess.
The -tohits/+def from Dark Miasma will likely give you time to fire Zapp without being interrupted.
Not an expert on PvP, far from it in fact, but Howling Twilight also accepts Slows.
I *think* Tar Patch doesn't stack anymore, if ever. Not completely certain, though. Mine's ready to fire as the old one vanishes.
As for Phase Shift, I'd suggest looking at the Ethereal Shift Temp Power Recipe. 5 x 30 second uses of a clicky phasing.
As for APPs, Electrical MacGuffins will give you Power Sink, auto-hit end drain to couple with Short Circuit. Not tried the combo personally, but I've heard good things. Charged Armor apparently stacks well with the Shadow Fall, too.