Can't you just use a bip85-like solution to derive a new "deterministic" mnemonic based on the nsec? 

https://github.com/bitcoin/bips/blob/master/bip-0085.mediawiki

If the nsec has sufficient entropy then you can safely derive a new private key on a new hardened path and use that as entropy for a mnemonic.

Since this mnemonic is derived from the nsec, this could solve your problem I wonder?

Where am I wrong? 🤔