Cant help you with the locknut key, but I have had sticking pistons in the past. Resulted in heavily biased pad wear, but no squeaking to speak of.
The stickiness came about as a result of the pistons having to be extended further and further as the pad wore out. The chrome on the piston then wore off meaning it would not retract back into the caliper. I tried everything to file them back down, pressing them back in with clamps etc, but to no joy. In the end I had to buy new pistons and then rebuild the calipers with new seals etc, before finally havign to bleed the whole system. Not a difficult job, just pretty time consuming!
Hope you sort it.