1) BE 2012 had a number of changes which means that as a prooduct some discontinued backup types, backed up with older versions of BE would also not be restorable in 2012
2) Also BE has never been able to do restores to a different version of Exchange than the original source (although if GRT had been used the the restore to PST option might have been available to you
2) As you have already found out, newer versions of Exchange do not support Brick level (which is kind of why we stopped supporting it as well)
The suggestion for duplicate the set to disk (I think) would only work with backup sets where GRT was used, as this the recovers the EDB files into the IMG folder, however you did brick level so I believe this idea does nto apply.
As such I think you have to build a recovery environment to get at the data
Which more or less means build a small AD environment (possibly offline from your main network) with the same organization names as the original Exchange setup and the same version of Exchange along with a media server running the same version of Backup Exec and then doing your restores (this is kind of an old school technique that a long time ago, before recovery stoprage groups and/ or GRT technology, was the only way to get back individual e-mails)
Once you have restored the data then use Outlook to extract the required e-mails to PST to move back to your production environment.
This will be a time consuming process (and may need some spare server hardware and the relevant software) - Good luck
