OK, here'* my guess. If the inner pad was OK, that suggests that the caliper piston was moving back in the bore when the brake was released, but if the pins were bad not allowing the caliper to slide, we would see more wear on the outboard pad. So, if that is correct, the problem was more likely to be pins, than the piston.
I'm thinking that failure to completely release, whether caused by a bad hose, sticking piston or failing booster, should show excessive wear on both the inner and outer pad.
But that'* just guessing. If there is someone on here who does brakes for a living, maybe they will help us out with a more informed analysis.
Oh, and while we are providing free analyses, I'd say (from the condition of the stitching) that the other shoe should be good for at least 6 more months, as long as you change your socks everyday.