1) You can use a Ledger Nano S either as a backup wallet of your Ledger Nano X (by restoring it with the same 24 words seed as your Ledger Nano X) or as a completely separate wallet (by generating a new and different 24 words seed). In both cases you must write down the seed(s) in order to be able to retrieve your crypto accounts in case of loss or damage of your device(s).

2) As far as I know, one CryptoSteel is for just one seed. Anyway you might purchase only one to try it, and later decide if you need another one.

3) No problem if this should occur as Ledger use the same seed standard (BIP 32) as other HD (Hierarchical Deterministic) wallets. This means that at any time you'd be able to import your 24 words seed in other wallets (even not hadware wallets) to manage your crypto.