On the last picture posted, there is a groove in the inside face of the left side of the caliper, that shouldnt be there, have you checked the discs? You also should check that the bobbins are free to rotate within the discs. Whatever size hole is in the bobbins, fit a screw with a nut on the back and nip them up on the bobbin. Then spanner the screw round while spraying in some brake cleaner.
Might also be that he pads are jamming in the caliper even though the pistons are returning. Check the brake pins/guides.
Failing that, full overhaul of the calipers. Not much difference in the end result of either riding around with brakes dragging and risk of locking up, or failing to apply when reqd