you are correct the caliper guide pin froze which is why the pad is worn only on one side, a common problem.
But you are not serious about re-using that caliper, you may have gotten the pistons to retrack but it is just matter of time.
Those pistons a FUBAR and and the dust boot is ripped also throw a used if not a rebuilt caliper on it.
as soon as the pistons start to move forward they will get to a spot and freeze, and you will be doing it all over again.

 

The heads that hold the calliper frame are 19mm 12 point. I know, I have just replaced my front discs and pads.

 

This is correct, I fixed the truck last week. a 12point open end box wrench was able to take the caliper brackets off.