That's precisely what left handed cobalt drill bits were made for.
The shop you to sounds like the lube monkeys got access to tools while nobody was looking.
Definitely remove the mount to get access, and so it isn't pushing on the remnant and helping hold it in place.
Absolute, worse case scenario:
Grind it flat, have a piece of flat bar stock welded to the mount to make it wider footprint, and it gets a new bolt hole and threadsert into the unibody/frame.