(1) If other people spot the seed phrase, they can take away all the coins. How do you keep the seed phrase long term?
The main thing is to keep the seed secure like your wallet.dat or private keys, preferably offline. The way I like to do it is to write it down on a piece of paper, laminate it and then keep it in a safe. For my uses, it is more than sufficient security to secure my seeds.
(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.
No. The seed phrase generated by HD wallets like Ledger usually conform to a certain standard and thus the method of deriving the private key is still open sourced and anyone can develop programs for it. There are still plenty of wallets using the same seed format as Ledger and it wouldn't be difficult to import the seeds into another wallet.