Brake Caliper rebuild / bleeding.... Removing piston questions... 5th gen

I have been experiencing brake drag since I installed my Galfer rear pads a few weeks ago... they were considerably thicker than the pads that were left on the bike... The pistons on the calipers pushed back in tough but I thought not much of it since I always used a C-clamp on car calipers... I put the brakes together and they were a little tight, but I figured after a ride or so they would loosen up after being broken in... wrong.

I pulled everything apart tonight... In the rear caliper, the 2 outer pistons move freely with the rear brake pedal as they should. I gather that the center larger piston should be activated by the front master cylinder on the caliper on the chain side... And it is... but boy I have to pump it 50 times to get the piston 1/2 way out... And then it takes all of my might (with a few rags over my thumb) to push the cylinder back in...

I gather this is not normal....

So I'm thinking that the rear caliper center piston needs a rebuild.

I'm looking for the best advice with the least amount of strife... I will only be putting new seals and a piston in the center piston... the other 2 look good and work good...

I'm hoping to work the center piston out with a vice grips and rag... If that fails I'm planning on blowing the center piston out with compressed air... If I pick the line that runs directly to the center piston I won't have to block the other pistons correct?

After doing the rebuild to the rear caliper is it really necessary to bleed the entire system or only from the 2 rear bleeders back? (first time doing this and it looks like a bear!)

Thanks!