Can a brainwallet passphrase be derived if an attacker has obtained the private keys of an empty wallet.dat file? Someone robbed me at gunpoint and took my laptop. The laptop contained an encrypted hardrive with a wallet.dat file. Both the hardrive and wallet.dat had at best weak passphrases for the encryption. I have assumed both to be compromised.

My question has to do with the one-way permutation hash function of ripemd-160 to generate both the private and public keys used in brain wallet generators.

[I am assuming that wallet generators are using one way permutations. I could be wrong. If I am then please inform me. Most wallet generators use the brainwallet.py available on github.]

If an attacker has obtained your private key can he "reverse engineer" the key to obtain the original brain wallet passphrase used when the wallet was generated?

If so I assume he can use that to bruteforce any other wallet that would use the same passphrase as a seed as mentioned in this post here:

https://crypto.stackexchange.com/questions/10246/question-regarding-multiple-sha-256-rounds-on-a-bitcoin-brain-wallet-passphrase
Sorry to hear you were robbed.

An attacker cannot reverse engineer a private key to obtain the original mnemonic phrase. 

What essentially happens is just another multitude of hashes with an algorithm (SHA256). Different approaches to this are taken by different deterministic wallet generators.

As SHA256 is considered secure. So is your mnemonic phrase