Thoughts gratefully received on how to sort this little sod out…

One of my bottle cage bosses is going round and round in circles. Bolt is (presumable) slightly seized in the boss and the rivnut/boss is loose in the frame. There’s not enough material on the edge of the rivnut/boss to grip it separately, and the top of bolt head is starting to round out.

I know how to tighten a rivnut up, but the problem is getting the effing bolt out of it to be able to do that. Any ideas?

I had to resort to drilling both the bolt and rivnut out then fitting a new rivnut. I tried everything I could think off including super glueing it but all failed.