yes, you have right only blockchain.info support could be help ... but they leave us alone, no help, no support for his OLD SHIT ...
and yes you have right my mnemonic is from before this new implementation - at that time the mnemonic = long passphrase was used by blockchain ONLY to decode/decrypt wallet id and password - has NOTHING to do with the private keys
https://login.blockchain.com/#/recover work for 12 words standard phrases
https://login.blockchain.com/wallet/recover-wallet work for STANDARD legacy wallet recovery phrases (although they are longer but need be standard)
EXPECT HELP FROM A OLD BLOCKCHAIN.INFO DEVELOPER OR SUPPORT .. shame him, that he leave a lot of his old = fisrt members alone with this problem
the initial set of words that blockchain.info gave its users had nothing to do with the key pairs the wallet generated, in other words it had nothing to do with HD wallets or BIP0-39 or any other similar key derivation protocol.
it was simply an account recovery phrase and as a result in order to get your account back you can only go back to blockchain.info recovery page or try contacting their support since they are the only ones who can help you.
try this page for recovery:
https://login.blockchain.com/wallet/forgot-password