This is just a guess...but is there a slot in the back of the collar where you can get a blade on the bolt? See if you can borrow a Dremel from someone with a thin cutting disc and just cut the bolt in half. Then take the collar (if removable) and the pieces to a hardware store and have them match it up.

It might take a bit with a Dremel and the thin cutting discs wear down fast/can snap if you flex them too much...so expect to go through 2-3 discs. Also tape off below/above the section so you don't cut the frame if it slips.

It's a huge pet peeve when I hear of stuff like this...the bike shops should have at least attempted some type of fix or gave you some possible options/ideas to do on your own.