Might try some penetrating oil and adding some heat - let it heat up/expand and add some oil when cool and hot for a cycle or two.
If you have to drill them, I would suggest a similar approach because you need to keep the bit small enough to not get to the threads and releasing some of the interior 'pressure" may allow for better expansion/contraction to free them up enough for extraction.
Good luck.
Thanks for your help, Trebb. FR is a supermind.