As to the stacks issues, it seems Kate and I have no problems during P1 (except that once, but that was...weird), just in P3. (Though, in Phase 1, I grab aggro "naturally" [without doing anything] right at 6 or 5 of her stacks - but only the first time. Subsequent switches require Provoke ...
The toggle wouldn't be suitable? Oh, yeah, didn't know if it worked with StA. Many people think the toggle is a bug of some sort. Heh. That should work fine yeah. In the alternative, can always wait till adds get close, then use Shellshocker. It has a 12 meter range, so not too bad. It seems to me ...
As far as I know, the passives are actually harmful (they don't reduce the blocker debuff on the rest of the group)... Plus the DPS loss would be staggering.