You have your “old” address of the “old” Blockchain.info (!) wallet - the one for which you have 19 words “like a seed phrase” and which, if you know the wallet ID in the format 12345678-90ab-cdef-1234-567890abcdef, and if you know the correct password, these 19 words are useless to you (hereinafter referred to as “wallet 2013”).
You are unable to import an account from wallet 2013 into your “new” Blockchain.com Wallet - the one for which you have a 12-word seed phrase, (hereinafter referred to as “wallet 2021”).
Thus, access to wallet 2013 is lost.
My next guess is:
To confirm login in wallet 2013 and wallet 2021, you are trying to use the same email address.
Or there is an option when for wallet 2013 there was only an alias and there was no need to log in to it after confirmation by this email (this is typical for the oldest Blockchain.info wallets, - until 2014 - confirmation to enter the wallet by email was not required, it was used just your nickname and your password).
Since Blockchain.com, and previously Blockchain.info, publicly state that they make archival backup copies of Wallet.aes.json wallets upon registration and after each use, then, therefore, these copies are available in the database archives of the Blockchain.com service itself .
Therefore, it is theoretically possible to request an archived copy of your Wallet.aes.json from Blockchain.com support.
If you can get the source file for wallet 2013, and if you remember exactly the password for your first wallet, then you can use BTCrecover to get the private key of your account, which you currently do not have access to.
So it’s probably worth trying to ask Blockchain.com support to find it in the archives and send you a copy of the source file Wallet.aes.json for wallet 2013.
Now I’m just trying to find out if any of the forum users managed to get an archived copy of Wallet.aes.json from Blockchain.com support. It seems that there have been such precedents before. And BTT users wrote that they succeeded.
But now no one has yet answered or confirmed that he succeeded.
A few clarifications:
The 19-word phrase is not a wallet passphrase. It is a mnemonic phrase, internal to Blockchain.info. Its only function is to provide access to the original password used to create an account on that service.
The 12-word passphrase they issued in 2021 IS a wallet passphrase. It passes the BIP39 checksum in Electrum, so it is a valid address. This was intended to replace the original 19-word mnemonic. Good on them for issuing it.
I have a copy of the original JSON wallet file that I downloaded as a backup in 2013, but I only tried to decrypt it for the first time this year, and the original password (which can be confirmed by using the 19-word mnemonic on their website to make them tell it to me) does not work. That is truly puzzling. In any case, I expect it would only tell me the 12-word passphrase anyhow? Which I already have.
I've used the same email address with their account since 2013, so that should not be an issue.
I think (I could be wrong) that what I need is more technical information on the wallet type and derivation path for wallets created in 2013. I'm hoping that if they share that information with me, I can finally access the wallet, even if they don't fix their website to the state where I can log in.