I've had that problem with stripped caliper bolts before...what I did to get it out was weld another bolt to the stripped bolt and turn the new one out.  Basically what you need to do is find a bolt with threads that will fit inside the 8mm head head bolt, weld it up, and put your socket on the new one.  A can of PB Blaster and a small propane torch are always good to have too.  As far as the caliper goes, use a C-clamp to push the piston in.  The piston should turn as it's going in. 
Brent