What if they removed the alacrity from staff ambush and added its effects of either Dune Cloak or Elusive Mind. The alacrity would still be on ambush but now you could provide alacrity with any weapon.
This might make you do too much damage now that you can take any weapon to be an alacrity support but since you wont be able to run Infinite Horizon at the same time, the change should keep you balanced.