So in hindsight, this firedrill you are going through, BackupExec isn't going to help you. However, with a signed affidavit as a "subject matter expert," you can pass off files with different date/times and present in an ediscovery request. I've done it, and have had to sit through a few depositions in the process to argue the technical validity of the process.
Going forward, if you are going to have to do this often. You need a real archiving product. One that not only can archive static files, but also search and discover active files. Autonomy, Mimosa, and Kazeon (Preferred) will do that. Not only can you search for what you need in network shares and down to the desktop, but they often integrate into backup apps too, to search in the catalogs.
Once you've discovered them and found them, you can then put a legal hold on them and store them in a repository for further review all while preserving the chain of custody.
