In A Pickle

Bolts should be grade 8 tensile strength, the thread diameter determines the size of the bolt, not the wrench required to turn it.
The broken bolt should be able to be removed using an easy out tool. Basically it means drilling a hole in the broken bolt, centered as close as possible, and using a reverse thread insert that once bottomed out, will force the broken piece to unscrew. If you are not confident in your ability to do it, any machine shop should be able to for a reasonable price.
I would then replace all bolts with new grade 8 hardware. If that pressure plate let's go at any point, you can lose a leg or worse.