No offense, but I don't see the point of this at all.

With your system, the user needs to back up an email address or other ID, a password, one or more "additional phrases", and a nonce. The loss of any one of these results in loss of their seed phrase and their coins. How is this simpler than just backing up a 12 word seed phrase which was properly generated from dev/urandom?

Either they are backing up all that information on paper, in which case they should just back up a properly generated seed phrase, or they are relying on their memory for random strings, phrases, and numbers, which puts them at a very high risk of forgetting something and locking themselves out of their coins.