[stephen low]

I struck a similar issue once trying to remove a head off a laser with the head bolts seized.

Application of heat and shock hitting the end of the bolts eventually freed them all.

Was able to set up a ring spanner and would work the bolt in both directions though a fraction at a time until the whole thread freed up. Got all the bolts out with no damage to the alloy threads that I was aware of.

Worst case though is as suggested, carefully drill out the centre until you can use an easy-out to reverse out the remainder of the thread. If you are down to this then thread damage is a likelihood but correctable too.

Good luck.

Steve