Is there any way suppress the automatic overlay of approach plates in navigation mode? Some of the plates are less up to date than the SkyDemon chart itself.

No, it isn't possible to turn it off in general.

I now realise that the reason for my not knowing about the red x is that by the time you are close enough to a destination for the plate to come up, the x at the corner of the plate is already out of view! I suggest that this disconcerting behaviour could be avoided by making auto plates optional and the default to no!