Recover keys from legacy blockchain wallet's mnemonic?
The legacy blockchain.info mnemonic does not correspond to any private keys. Private keys are not derived from that as blockchain.info did not use HD wallets when they had those mnemonics. Rather the mnemonic is an encoding of your blockchain.info password.

In order to get your private keys, you will need the JSON file as that is what contains the private keys. If you know your password, the mnemonic is useless. If you don't know your password, get it using your mnemonic.

Unfortunately I don't think you can use blockchain.info JSON files outside of blockchain.info (at least not easily). You can try importing it to a new blockchain.info wallet using this: https://blockchain.info/wallet/import-wallet