I'm wondering what's involved and or if the old wallets are even accessible anymore. It was the original wallet version from the first year of bitcoins launch. Seed phrase and then the other password and so on.
Sounds like you're confusing several wallets into "one"... You talk about "wallet.dat" which is Bitcoin-QT/Bitcoin Core but then mention "Seed Phrase and then the other password" which sounds similar to the old blockchain.info recovery phrase and "second password". By "wallet file", perhaps you're thinking about the backup wallet file that you could download from blockchain.info?
Note that "Passwords" (terminology used with Bitcoin-QT/Bitcoin Core is "passphrase") were only added to the wallet.dat in version 0.4.0 which was released 23 September 2011. Refer: https://bitcoin.org/en/release/v0.4.0