OK I’ll answer some of this myself, please find attached a Layers Experiment.
The Order of the Layers is important even when not in Edit Mode, as it affects what is Visible and what is NOT Visible.
As far as I can work out, Layers at the bottom of the Layer Editor window are in the Foreground and Layers at the top of the list are in the Background, so if you have Modulators in the same physical X, Y space then you need to make sure that any modulator which needs to be visible on top of another is lower down in the list, closer to the Foreground if you will, or it could get hidden behind!
For instance with this panel attached if you move Layers 3 or 4 up higher in the list than Layers 1 and 2 then they end up getting hidden behind the tabs (which are on Layers 1 and 2).