plug change
I think I would try and prise out the bolt complete with the brass ferrule or at least until I could get enough purchase with a long nose pliers to hold the ferrule and slacken off the bolt, you could also try gripping the head of the bolt with "mole grips" or locking pliers and pull it out.
When successful either get new ferrules made of a slightly larger outside diameter and knurled or replace the originals with araldite and don't over tighten the bolts.
Hope this helps.
Nigel
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