Ok. An additional word to the standard 12 or 24 will simply generate a new mnemonic, a new set of private keys. What are actually the mnemonic words? A human readable form of private keys.

What you describe is practically a method of plausible deniability. Let's say you save those 12 words seed in 2 places. Then to one set you add a 13th word. This will be your real wallet with funds. Then to the same 12words seed you add a diferent 13th word and save it in another place. As a decoy. As a test. Empty or just with few meaningless sats. Or much better deposit a large amount of BTC but later move them from that wallet to another one, to seems like a "hack".

If you are in a threatening situation you just give them the decoy wallet, emptied and play the role of " hacked" loser that lost all his BTC.

In regards KYC, say KYC identity or KYC exchanges. Why? Because if you say kyc bitcoin that means there are 2 types of bitcoins. And that is not true. If that will happen then bitcoin is dead, useless, worthless.