I snapped the cable adjuster while trying to adjust it, shearing the bolt right off at the derailleur. It's an old 6 speed Shimano SIS derailleur so replacing it should be pretty cheap. I hate to toss it though. Is there any way to remove the sheared bolt without destroying the threads on the derailleur?

If I can't successfully remove the bolt, will most Shimano RDs work with a Shimano 6 speed indexed shifter? Such as an 7 or 8 speed Shimano RD with a 6 speed shifter?