I have a loose rivnut with a bolt still in it (on a 2004 Cannondale aluminum frame) and every method I've tried can't seem to get it out. I've tried lodging flathead screw drivers and diagonal pliers between the rivnut and frame but they just can't seem to get enough leverage to spin the bolt loose. I've tried super glue but that failed as well.

Took it to a shop and they mentioned that they wouldn't touch it (I'm assuming its fear of damaging the frame) and that any method they would try would not be good for the longevity of the frame). My question is if there is any truth to that last statement? If I were to angle grind the bolt and file it down carefully, will there be any ill effects to the frame?