Regarding #4 (spare cell phone battery)

Rechargable battery packs tend to self-discharge (that is, they discharge whether or not you use them) on timescales of a week or two. If you simply carry a battery with you for weeks or months, it will almost certainly be dead (and certainly not at full capacity) when you need to call it into action.

The solution I use is to keep switching back and forth between the two batteries - I use one, while keeping the other "topped off" in the charger. When the current battery gets low (below half), I start to carry the spare with me, and swap out when necessary (the dead one goes into the charger to remain until it's needed again in a couple days).

Alternately you could keep one battery in full use, and "top off" the backup every couple of days, but I'm unsure whether that'll have adverse effects on battery life. The manufacturers say the memory effect problem has been solved, but in such an extreme case (always top off at 90 % and never properly use), I'd still worry that the battery won't be available when I need it.