Yuk. You're in for a hard time regardless of what you do. Good to see you are trying though.

I don't think cutting the head off will help any other than giving you a little more room to work. You won't be able to slide the caliper off by just cutting the head off the bolts...the rotor will prevent you from being able to "slide" the caliper off the studs left after cutting the heads off, if I understand your predicament.

I concur with George. You will probably have to drill out the bolts and then use a screw extractor. This is cumbersome and a pain, but it will probably work. I don't think you will "botch it" if you are careful and step up the drill sizes gradually.

The rest are my opinions;
I'd replace only the caliper that was no good;
I would not replace the rotor unless is was deeply scored. If it did need to be replaced, I would do both. (I have a set of new Brembos from The Parts Bin that I never used);
Parking brake shoes / hardware I would replace in pairs ie. both sides.

Remember, this is fun!
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'60 544, '68 220S, '70 145S, '72 144E, '86 745T