[Windows Phone 7 Q&A, Help & Troubleshooting] GoodDayToDie:
I don't think you can "trick" it into seeing it as your Focus, because from a software perspective, it just isn't. However, the restore points (backups) are tied to the hardware, not the software. To see if you can restore one, first check in Window Explorer at the path %localappdata%\Microsoft\Windows Phone Update\ and see how big the contents of that folder are. There should be hundreds of meg to a few gigs, depending on how big the backup is.

Now, that's only going to be the most recent restore point. If you made backups of your earlier restore points, you can send the phone even further back, hopefully to before quite so many things get messed up. However, each time you do an update with Zune it overwrites the location where the previous restore point was. Note that if you installed any updates through a mecanism other than Zune, for example usign the WPSHIFT tool that was used for the beta, it probably took a backup but might have put it in a different location (often in TEMP, which hopefully you haven't cleaned out).

If you have Volume Shadow Copies enabled on your PC (Vista/Win7/any recent Server), you may be able to use the Previous Versions feature to pull an older restore point than the most recent one, or to recover a restore point if it was deleted for some reason. Be careful, though - old shadow copies are deleted as space is needed for other things, but they aren't deleted atomically; it's possible you'll pull only half of an old restore point or something equally silly (they're made up of hundreds or thousands of small files).

Once you've got your restore point, I strongly recommend making a backup of it by copying the folder! Then, connect your phone and start Zune, and go to Settings -> Device -> Update. It'll probably say there aren't any updates available, that's fine. In the same part of the window where it says that, though, there should be an option to restore a backup (it'll give the date of the backup and a restore button). If that button isn't visible, re-copy the restore point over the location where Zune expected to find it (%localappdata%\Microsoft\Windows Phone Update\), and check for updates again.

As for the un-Walshing tool, it probably won't do anything on your OmniaFocus. It may fix the problem if you successfully use a restore point (and get back to a pure Focus), though. There's a good chance that the last restore point you have will be from right before you began the Mango update process, in which case your phone will be back in its Walshed state, ready for un-Walshing using the tool.

I hope that helps. If you do decide to try flashing again (if you don't have a restore point, this is probably the only option), make sure you're using a ROM for your hardware revision of the Focus. There are two, the 1.3 revision (the first one released), and the 1.4 revision (released later, identical except it uses a different Flash memory chip internally and therefore has different drivers). If you bought your phone any time in 2011, it could be either a 1.3 or a 1.4. There are a few ways to check, but one of the best (since your software is currently messed up) is to look under the battery. There's a sticker that should say soemthing like "rev1.3/rev1.4". You need to match that revision number when selecting a ROM to flash, or (even if it works) you wouldn't be able to access the phone's internal storage.

It seems a little unlikely to me, but it's possible that Walshing your phone made it report itself as an Omnia, thus the reason you could flash that ROM (but not the Focus ones). I'd expect that to be based on lower-level hardware checks, though.