Hello John,
The magneto points generally aren’t a problem once they’re set, although the assumption of contamination getting on the retard breaker points is one possible cause of your problem. More often, I’ve seen breaker point hold down screws that weren’t torqued properly or the retard breaker point wire getting pinched between the magneto case halves (there’s a lot crowded into a small space on that left magneto.)
I had to look at the switch wiring schematic to understand what your mechanic was talking about with that wire jumper- but I think he’s really onto something there, and it seems like it should work to isolate the magneto from a potential switch problem. I think he knows his stuff.
The reason I still lean toward a possible bad wire is that those things lead pretty hard lives. Vibrating on one side, anchored on the other, and hot as blazes most of the time. If the wire was starting to break down, it could have been helped by the magneto removal, and then over a few starts shorted again. If nothing else, it’s pretty easy to check once you have the top cowling off.
If you do send the left mag back out, Aircraft Magneto Service in Montana gets very good reviews and seem very trustworthy
Good luck, please keep us posted.