Hi, okay so we have a few tricks on our boats for this sort of problem. The thing that may well be grabbing your bolt is aluminium oxide. This has a habit of expanding in the thread and hold on really fast to the bold inside. We found two ways to deal with this. One is heat, carefully applied to the bolt. Not just once but several times. Don't go mad with this method as the bits you are dealing with are ali and will lose their properties if you over do it and then need heat over the whole piece to anneal it afterwards.
Next is a mixture of hot water and baking soda applied several times. This is really what the the plating companies use to strip jobs that have gone mammaries up. (in some places) It will assist in dissolving the ali oxide that is grabbing you're bolts etc. Above all patience is needed as it has to dissolve the said crap around the tread.
Brut force normally ducks up the thread if the bolt holds.
So, heat, strong caustic soda and boiling water, plus a good penetrating oil or electrical contact cleaner.
It will come out, just how is the bit that matters.
Hope that is of some use. 40 years at sea with fkin ali
but that's another story. 
