OK, I finally have some more news to report on this board.
I've been probing all around over the past couple of weeks when I had the time (which was a bit less now that the holidays are over and I'm back at work

). Can't say that I found anything new...
So I hunkered down with the lighted magnifying light and did a thorough scan of the board, both the solder and the component sides. I found some traces in the area of the RAM chips (N2, P2, etc.) on the solder side that were exposed and did look like they might have been scraped by something, tested them all, and they seemed to be OK. Nothing seemed to be broken, and none seem to be touching the other. But I cleaned them up anyway. This was a couple of days ago.
Then I had some time today and plugged the board in with the intent of doing some more probing, and voila! The board came up OK!
There was some bad bowing on the top, but I replaced the M1495 chip at A/B13 which took care of that, and now the game has a great picture and is playing perfectly.
Wish I could say for sure what fixed this puppy in the end, but I'm guessing it was the funky traces I found and cleaned up.
Hopefully, it will stay working, but I had it on for over an hour just now, and it seems fine.
Keeping my fingers crossed

!
Jon