FileVault 2 Encrypted drive was reformatted by thief then returned. Is the original encrypted data salvageable?
I'm afraid there are not many options given an ordinary, "home user" budget.

Theoretically you could send the drive in to a recovery service such as IBAS to get them to recover the original data on the drive. If this is even remotely possible depends on what kind of physical medium you had (i.e. is it SSD or spinning-platter hard drive). If they can recover everything, you can use the standard Apple tools to access the encrypted drive with your password or recovery key.

If you can't recover the full data set, you would have to inspect the data manually and probably create new tools for decrypting blocks of data using your recovery key. This will require specialized knowledge and skills, considerable amount of time - and with no guarantee of success. It is out of range for ordinary, home user purposes. FileVault 2 uses AES-XTS encryption - if you haven't got the recovery key and/or volume master key it is going to very hard (next to impossible) to decrypt.