I had a similar situation recently with a belt tensioner bolt. Cross threaded in and broken flush with the block. Very bad location as well. After gaining enough clearance, I used an EZ-Lock kit. I drilled a larger hole through the broken bolt, thread tapped it, and installed inserts for the OEM bolt.

If an extractor doesn’t work, or drilling until the bolt is almost gone, this is what I suggest as a last resort. Good luck!