Prius C rear license plate bolt rusted

My niece has a 2012 Prius C. One of the rear license plate bolts is rusted and cannot be removed. The bolts are Phillips head M6x1.0mm and about 1-inch long. The steel nuts are apparently spot welded to the inside of the hatch. I tried spraying it multiple times with penetrating oil and using vice grips, but it's rusted solid. Finally, I cutoff the old plate and have the new temporarily attached with one bolt.

I was thinking of just drilling the head off the rusted bolt and trying to unscrew it from the inside using vice grips. My question: Is the bolt accessible from the inside of the rear hatch after removing the fiber cover?