What happens to my m/n Multisig setup if I lose one of the n seeds? Like, if I have a 2/10 Multisig and destroy one of the seeds, do I end up with something like a 2/9? Just need to know if n-x > m and I have all the xpubs and derivation info.
Nah, it doesn’t turn into 2-of-9. You need all 10 seeds for wallet recovery, even if you’ve lost one. The 2 keys you can use to sign are still part of the original setup, but losing one seed doesn't change those requirements.
I think the 2-of-10 setup would give you a larger transaction size than 2-of-9. Just having one less seed doesn't change the original multsig structure.
As long as you still have metadata, losing one seed shouldn’t completely wreck your wallet recovery. But it does reduce the security a bit. So, with a 2-of-10, losing one seed means the security risk for the other cosigners is pretty minor.
If you have the xpub for those co-signers, you can recover others without needing the lost seed. Just create another multisig wallet to move the funds because that lost seed could make your wallet less secure. If someone gets one co-signer's details, they might only need one more to compromise it.
Technically, each address in your 2-of-10 Multisig is a unique script with all 10 public keys included. If you lose one key’s seed, the wallet still needs that public key to open up the script. Losing it changes the script completely, which means you can't treat it as a 2-of-9 setup.