Yes, I would have used a seed extension.

What makes me question the one-way'ness of mnemonic > seed > xpriv are the functions mnemonic_encode and mnemonic_decode in https://github.com/spesmilo/electrum/blob/master/electrum/mnemonic.py.

If it's one-way, why can you freely en- and decode between the two?

But yea, mnemonic_to_seed does salt and hash the mnemonic.

The thing is, that with all the shitcoin-hardforks of bitcoin, you always have to handle individual private keys. And while I'm super caution to never let a private key and the master public key touch the same physical machine, I'm still paranoid. Especially about shitcoin clients, which may unannounced to me, leak the private key to a 3rd party, that then just waits for the corresponding xpub to emerge one day and cleans out my wallet.

I went for option B, and created a new seed.

Thanks