What about people with missing fingers, hands?
Can those people be immortal? Or are they doomed to age because they can't use the rings?

they need Biotek to regrow fingers.. otherwise they are doomed but maybe they can try with magnets on hands near the little fingers

What if they're missing an entire arm?

they can use foobraces and a magnet around neck. And maybe magnetic coil.

Oh no don't do that! That's harmful. Don't do that. You don't want magnets around your neck, and here's why. It makes explosions throughout the body. Now that's a bad thing. You don't want explosions in the body. Franco, bad advice. Bad Bad advice.