I ordered a spare battery and an OpenPGP card.
Rationale: Free shipping if you order with your phone. It may not be possible to get a Lithium battery shipped separately (for safety reasons - will depend on country and carrier) later on when I really need it.
I have some experience with each of the two accessories that I ordered - to the extent of establishing that they ‘work’. (There is still much for me to discover about how to use the OpenPGP card.)
Most definitely. I am doing it.
The SIM card and the uSD card go in the tray on the left hand side (a fairly conventional arrangement, at least as far as the SIM card goes).
The OpenPGP card goes in an internal slot, accessed by removing the back cover and the battery.
If you literally keep it in storage then I think there is a high risk that it will not work at all by the time you need it.
I am managing that by periodically changing the battery roles. Since the batteries have a unique number it is somewhat possible to keep track of which is which. I am not swapping out daily.