For example, use https://github.com/StricaHQ/bip32ed25519 to generate/convert the seed phrase to a private/public key.
@enzo to generate offline addresses from mnemonic, you can use cardano-addresses https://github.com/input-output-hk/cardano-addresses
You can do it using cardano-cli and cardano-address (or cardano-wallet instead of cardano-address, with the same parameters) using the script here: https://gist.github.com/cardano-apexpool/99784308812bdf7aa01446fa95f47fd2 I've been using it a lot in the last year.
Yes. I used the Mnemonic Method detailed here:
You'll need cardano-wallet on an air-gapped machine.