Wot shark attack said.
I did something similar a few months ago on my carbon road bike, tried to tighten a loose bottle cage bolt and it was seized. Cut the head off (between the cage and the frame) and then panicked! Tried cutting a slot into what was left of the bolt to unscrew it but it was corroded in place and I made a right mess.
In the end I drilled it out and it was much easier than I thought it would be. Even with the mangled half bolt still there the drill bit was pretty much guided by the old rivnut so the inner popped out and into the down tube, the outer ring of the old rivnut came away with a gentle nudge.
Took the fork out and rattled the old rivnut out, then fitted a new rivnut with a little epoxy between the rivnut and the frame (recommended but not critical). End result was good as new, I was tempted to drill out all the others and put in new but decided not to temp fate.
Long story short – pretty easy one to fix yourself.