Recover keys from legacy blockchain wallet's mnemonic? I have an old blockchain wallet that is worth trying to salvage

It seems the old password that the 18 word mnemonic originally had isn't working and another password I had listed also doesn't work.  

I used their tool and it shows the original password.
I also have the wallet backup json file.
I have the email address for this account (I see all the notifications)

Can I derive the keys from either the mnemonic or json with the original password or perhaps with no password?

And has anyone be able to get blockchain to 'backup restore from an earlier date' like it says they can on their legacy mnemonic page?
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