I never thought about adjusting the fly back.. interesting idea
On the Plus, there is no adjustment on the flyback, so I'm guessing the G2 voltage is the cut off voltage controlled by R53. I could be wrong, but I didn't see any other adjustment on the flyback itself. And it made no difference. I also re-soldered all the pins on the connectors to the yoke, logic board, and CRT neck (not sure what it's official name is. No difference. I've got a U2 replacement on the way and will let you know if it makes a difference.
The one thing I have confirmed on mine is that it is the analog board. I can swap out the logic boards and get the same issue. And If I put in another analog board on the same CRT / logic board, and the issue goes away.