In terms of survival bags / shelters etc, I think it depends on intended use.
Is it something you plan to put up and take down routinely, as a regular shelter?
Or is intended to remain packed, for genuine emergency use only?
I don't have any of the lightweight shelters, as that's not part of my general use scenario right now.
If I start to make longer multi-day ski tours, then that will likely change.
In that case, I'll be carrying a reasonable amount of kit anyway.
If I'm in hill-running mode, I will have a very small foil bag which permanently lives in a dedicated pouch in my hill-running vest. These are typically Required Kit on any serious hill run events. They are single-use items, pretty much.
Regarding nav: Many Scottish hills, in particular the more Easterly ones, can have fairly large featureless plateaus which can become very disorienting in poor viz. More rugged terrain with well-defined ridges can be easier to follow. If the viz is closing in, you need to determine your location NOW, before you loose sight of any points of reference. A map and compass are useless if you don't know where you are to start with. GPS greatly assists with this, and my watch is configured to show me an OS 10-figure reference. Tracks on the GPS are great too, but the ability to determine your current location (and elevation) to use with an actual map is the main feature.