So I like to hear and share the 'rest of the story' when I post for advice. I figure it's my duty since others took time to educate me and offer suggestions. Here's the rest of the mag story: First, I bought a new wiring harness.
We were confident this resolved the issue because we'd previously cleaned up the distributor block finger but the problem continued with the #2 cylinder. However, $600 wasn't enough to appease the aviation gods. The engine still had a noticeable roughness that got worse with RPMs. This warranted further inspection, really honing in on that distributor block as the only other reasonable culprit.
As a reminder, here's what it looked like before:
. After removing the mag and cleaning the block up really well, this is what we were looking at:
. So off I went to get a new one. A&P installed it and hopefully this has resolved the issue. She ran right through all RPMs while testing. It was interesting and, besides the part about dropping 1.5 AMU on the repair + labor, fun learning something about my plane I didn't previously understand. For example, now I know how the left mag fires:
. After this episode, I also knew how to steal a 172 before anyone posted the answer ;-). Anyway, hope this is helpful for any owners like me who may not be overly mechanically inclined or didn't grow up on a farm where working with this technology may be a daily occurrence. At 53 I'm still learning & that's OK.