Work thru your scenarios and some things become clearer.

For instance- the signal mirror question.

It's a signalling device. Is it the ONLY means you have to make a signal? (probably not) Is it the ONLY kind of signal that will keep you alive? (as in - somebody will call in artillery fire on your position unless you give them a mirror flash) Probably not.
Is there a specific somebody at a time/place that is expecting your mirror flash? (probably not)

So if it's not all that important, and can be substituted with something else (such as a smoky fire, a whistle, a flashlight beam) then it's not important enough to carry two.

I'm not a believer in the one-is-none, two-is-one mantra.
Only for SOME things. For instance, when I go flying, I have a spare pair of eyeglasses within reach. (w/o glasses I couldn't find the airport)
When I go hiking, I have extra warm clothing but only 1 GPS. My one compass backs up the GPS if needed though situational awareness is my real backup. And so on.


BTW- I'm a Reliability Engineer.