Time for new Calipers - just remove old and install new. You are paying now for your lack of proper maintenance in the past.
and the Caliper pistons and Cups are old and will soon leak ruining all the other work you are doing. Do your BRAKES right - too much at risk to do a 1/2 way job.
It’s a used vehicle purchased not too long ago, in overall exceptionally good shape — it wasn’t MY lack of proper maint, and not really even the prev. / orig. owner, as the maint. records look good: It was just some gorilla in a shop, as other have noted...
BTW, “ALL”,the main bolts that attach the entire caliper assy. to the vehicle are locked too: Had thought of removing the entire assy. to get better access to the caliper pins, but, no such luck.