As people have stated previously, stainless steel galls easily; it also has a tendency to work harden, making drilling difficult. Some vacuum hardware utilizes silver coated threads to avoid this problem. For example, the threads on VCR fittings are typically plated with silver. I don't know what machining facilities Mizuno has, but if he can get hold of a vertical mill and carbide tooling it should be possible to either drill or mill off the damaged hardware (assuming it is nuts/bolts). I have some experience machining stainless, and the combination of rigid a setup, carbide tooling, and appropriate coolant works well.
I would just attempt to loosen the bolts with the best wrenches available and twist until the bolts break. I used to do that all the time. It doesn’t take much torque to break the bolts. Replace the broken bolts, each as they are removed, with new ones tightened just enough to do the job if the seal is desired to be maintained. iMHO.