(1) If other people spot the seed phrase, they can take away all the coins. How do you keep the seed phrase long term?
This is not troubling, you don't allow someone to see those phrases. You must keep it elsewhere that you alone knows where it can be found. Write it with various backups and much better to avoid backing it up with any software on your computer or smartphones. Just plain handwrite.
(2) I read that "generally a seed phrase only works with the same wallet software that created it". What if that software - Ledger Live is no longer available or workable in the future? Thanks for any relevant info.
AFAIK, you can access those phrases with any wallet except webs.