Why is this 12-word mnemonic deemed invalid by multiple wallets?
Blockchain.info did not use HD wallets or BIP 39 in 2013. Their change to HD wallets and BIP 39 mnemonics was fairly recent (within the past year or two IIRC). The 12 word mnemonic that you have is not a BIP 39 mnemonic. Rather it is a custom mnemonic used by blockchain.info for recovering passwords. The mnemonic itself encodes the password to your account (or information to recover the password from blockchain.info). It can only be used with blockchain.info and their legacy wallets.