[Bibio]

from what i remember the bolts are stainless steel so you would need to use TIG to weld a nut on. personally i would just bite the bullet and drill it out going up in stages till only a little meat if left then wind it out hopefully leaving the thread intact. dont use cheep bits buy proper SS bits and SS cutting paste or you will blunt the bits. there is a cheat... once you get a decent hole in the bolt thread you can whack a star bit into the hole and use a windy gun to do the rest.

as for the copper bit then yes it is a gasket which means the original ones have been replaced at some point as the original ones are aluminium sandwich.

if the bolt is just turning in the captive nut then just punch it out from the back instead of trying to cut it out.

the gasket in the mid pipes are collars and if damaged then need to be replaced.. kerching.....