As for the timestamp argument, asking for all of them from the last 2 weeks each time you spin up a client isn't going to be signifcant even though you are getting repeats.  And if new backdated ones flow in while the client is up, they will come through that same subscription after the initial EOSE without having to ask again. So repeats only happen when you start up a client.

I should correct my earlier post, it was NIP-59, the giftwrap NIP, that would have benefitted from the insight that you could dox your DM counterparty without exposing your nsec.

Kind 14 is essentially Kind 1 used in this context, except maybe the different kind number helps something. We could have a couple dozen new kind numbers that didn't mean anything different from kind-1 and it wouldn't hurt.

I don't personally like seed phrases. They are ok for recovery, but not for login.  For login a user wants to remember a passphrase that they get to make up from whole cloth.  There are algorithms to map that into a seed phrase.  NIP-49 goes from a passphrase to an nsec.  nsecs being 256 bits probably can be mapped to a seed phrase too.  Everything with sufficient entropy could be mapped into anything else.  Just saying there are a lot of possibilities here.