If your main concern is getting the broken bolt out... most likely you will need to drill the bold and use an easy out unless there's enough above the surface to get a vice grip on it and apply some heat to the knotter frame. If you have to drill it... I'd probably remove the whole knotter stack from the baler taking VERY carefull note of where all those spacers go exactly. Then you can get clear access to the knotter frame to drill it. I would not disassemble anything beyond that unless it's absolutely necessary and that includes leaving the twine discs alone.
The sad part of this saga is that you probably didn't need to do any more than adjust the tension on the twine holder spring... and didn't need to touch that arm at all.