Can you just probe directly to each glow plug to read the ohms. Or scan each of them for temperature after a glow cycle? If there is any confusion on the wiring.

Since they are not the series plugs it is only important to separate the common power feed to all of them first for a resistance check.. Sounds like this may not be practical. So if you do not have a thermal temperature reader perhaps borrow one?

I hesitate to suggest touching the ceramic portion of a glow plug right after a cycle with a wet finger. As I have no idea of how hot they would be. I have never used a thermal reader either after a cycle on them. If we found they are really not that hot the finger test should have evolved by now. This suggests they really may get too hot externally for this as well. The threaded post from the element to the external nut should absorb at least some heat. From the white hot element.

Sounds like a very good find. I would not be absolutely certain it is a glow plug problem. Although it usually is.

Or just read the seven connections with the ohmmeter. If you find there are five similar readings is all you are interested in. Without knowing I doubt the sensors read similar values to the glow plugs. Although with my limited knowledge they might.