is this rear caliper salvageable?

[His Dudeness]

I'd reattach the caliper, bleed it and clamp the piston that's moving and then pump the stuck one out a bit, then clamp it and pump the moving one out a bit and keep alternating the clamp until both pistons are out. You have to pump them out evenly or one will get stuck. Then as deefer says clean and inspect the seal recess and decide if new seals will work. I'd buy Yamaha seals, after market seals can be a bit hit and miss.