Seems this hole just keeps getting deeper and deeper.
I'm using DiagROM 1.2.1
Regarding booting into ECS or OCS, I see no direct option for these, but I guess you are referring to "original" under the "display" option in the boot menu? When booting using original, the game Pang for example just goes to a completely red screen when the level starts where as enhanced or best boots in normally to the game but with the game glitches as described before.
I guess I'd just expect different (more random) bad results if there is a physical connection that is broken, but I really have no idea, its far from my expertise.