I finally figured this out, but thought that I would post it here just in case someone has a similar problem. My Medieval Madness (original, not remake) has always had an intermittent problem with the Merlin hole not kicking out the ball. It will go for several games without a problem, then it won't kick the ball out during game play. As soon as the game goes into ball search it will kick the ball out first time, every time. Turns out the problem was a missing rubber bumper at the bottom of the kicker (part #5 in the attached diagram). Because of that the coil plunger when at rest was too low.
I'm not sure how I missed this the first few times I attempted to troubleshoot the problem. I guess I was just too fixated on it being an electrical problem. And man, I did about everything conceivable to correct that non-existent electrical problem! New coil, new connectors, new connector headers, replaced fuses, tested every related component on the driver board, even ran new wires directly from the coil to the driver board; you name it, I tried it. Oh well, at least it's finally fixed. Had it kick out 100 balls in a row to test it, zero problems.

Merlin (resized).png