OK, interesting - so the 24-word seed isn't propriety to Ledger somehow, and we can use it to derive the Pkeys. Good to know. If I did that on a permanently-offline computer, should be fine, right?
A mnemonic code is part of the
Bitcoin
Improvement
Proposals (BIP 39:
https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki). Its not associated to ledger in any way.
Yes, if you do this on an 'permanently-offline' computer and noone will have access to this computer, it should be fine.
I would still recommend to move your BTC's out into a new wallet before claiming all the
shitcoins altcoins.
Regard your seed as compromised from this moment on.