It sounded like N'hala moved, not shifted, so Belkar could take an OA anyway; they don't have the same limits other Immediate actions.
We don't really want anymore reinforcements. I think our current goals should be getting rid of the staff - the boss is hard to reach so it might be better to use ranged attacks if possible.
Since those gates are still open, we might be able to push or slide the new aboleth back through them to get rid of them once the staff goes away. That might be the fastest way we have to take them out of the picture.
I was thinking of moving to T9, and then probably firing another Sacred Flame at the new lasher. I could also aim a Break the Spirit at the new Slime Mage, but the difficulty in moving is really making R10 a severe limit.