I've only used one Nikon in my life, I don't remember what model it was, but it wasn't manual, so I can't help you with your FM-2 questions.

But I do like your way of thinking on the manual camera for a backup. My favorite camera is a Pentax ME Super, it can be used without batteries, but has limitations. I always wanted to find a nice cheap K1000 for a second camera or back-up that would take the same lenses, but never found one cheap enough. I did find a great deal on a used Pentax ZX-50, but I hate that camera. Anyway, a couple years ago my son comes over with a Sears T L S body he'd bought for $5 at a yard sale. It needed a thread on lens, which I happened to have one, and the light meter didn't work properly. He lost interest at that point and gave it to me for my camera box. Out of curiosity I loaded it up and took it out with my Sekonic ambient light meter and shot a roll of film. Wow! Now I really wish I'd had that K1000 when I was more active in the hobby.