Guess I need to read your whole story on this bolt hole as I cannot understand what's going on LOL
come to think of it, if what's stuck i there is made of non metal, you could in stages drill it out right up to the size drill bit that would be used before tapping that thread size, then actually use a matching tap, and work it carefully.
Going a little and backing out to remove debris, then go again.
Be sure to use tap fluid. I've been doing a lot of tap and die work recently customizing serious things such as a mount for my flood lights. They are hanging from a 1/4" aluminum strap that is mounted to the underside of the Hannibal rack.
What's cool is there is a slot channel on the bottom of the 4" wide slats similar to the top channel so I used Thule slot rail plates which I threaded into for hanging the 1/4" flat stock.
Then I bolted the 4 Hella lamps from below to that using M5 or 6, with nylon lock nut on top. This arrangement allows left to right adjustment and the lamp itself has up/down angle adjustment.
The whole 4 lamp/mounting strap can be removed now as a complete unit in about 5 minutes. It sits above the sunroof glass but still allows full movements of sunroof. It is however set back far enough to also be protected from branches, etc.
Better detail photo tomorrow.