Pretty sure you need to check the master cylinder. Could be something else silly, but really can't think of anything inbetween.

The issue with the one pad, on one side wearing more than the other, definitely sounds like the sticky piston (which you've sorted, and noticed it was sticky when you looked at it, yeah?) If this piston is DEFINITELY no longer sticky, and no others are, then I suppose it has to be the master cylinder and this single caliper is simply an additional issue caused by a lazy cylinder return.

Hope you get somewhere with this, annoying problem for even simple driving.