But I assume you still can't use the restore deterministic wallet approach (which was my question), as the Mymonero login uses 13 words, vs 24 for simplewallet?
One word is a checksum. The other 12 words are supported by this:
https://github.com/monero-project/bitmonero/commit/2e11eb150444086dd936b10680bf354b548cc1f5And is there any difference in the two restore approaches?
No a wallet is just the keys file, and the words are just a mechanism for getting the keys.
Also, by the use of the phrase "deterministic wallet", I figured simplewallet worked similarly to Electrum, and thus multiple keys / addresses could be generated and used by the wallet, and the restore deterministic wallet would mean all of those would be restored by the same 24 words. If this is the case, does the Mymonero wallet use the same approach, or for simplicity, does it just use one key / address?
The generation of one-time keys for each transaction using the original (wallet) keys is an inherent part of cryptonote ("unlinkable" i.e. stealth addresses), and is described in the white paper (section 4.3).
The word deterministic here refers to whether the keys (wallet keys, not one time keys) themselves are created using a RNG or from seed words. Using seed words allows the key generation process to be repeated by providing the same words, resulting in the same keys.
