"Updating" to an older version is not possible, no. It might be possible to spoof the current version to an old one, if the phone is interop-unlocked, and then "upgrade" to a firmware release that's actually older than your current one... but even that might not actually work, if the updater checks the checksums of the "old" files. It wouldn't work with OS updates for sure; those contain differential (rather than canonical) patches.
Installing an old, stock ROM with the old firmware does work. However, while this is possible with Samsung gen1 phones, I don't believe anybody has found a way to do it with gen2 phones yet.
Finally, restoring a phone backup (as made by Zune when updating, or as made using the Easy Backup tool which just spoofs the update process), definitely does restore both your unlocks and your ability to unlock - that is, it restores the registry (where the unlocks are controlled from) and it restores the firmware (where the ability to unlock is implemented). If this didn't work for you, it's due to one of two things:
A) Your backup (more properly called a "restore point") was created at a time when your phone already had relocked and lost the ability to unlock.
A.1) You can use an older restore point, if you kept them (which I strongly recommend); just move it back to %LOCALAPPDATA%\Microsoft\Windows Phone Update.
A.2) Before you ask, no, you can't use somebody else's restore point; they have device-specific encryption that we haven't broken yet.
B) Your restore worked, but trying to interop-unlock again failed because you updated your Diagnostics app from the Marketplace.
B.1) You can undo this by first uninstalling Diagnostics from the app list, then re-installing it using the phone dialer.
Win8/Windows RT Projects:
List of desktop apps for hacked RT devices
XapHandler, Root Webserver, OEM Marketplace XAPs, Bookmarklets collection (Find On Page), Interop-unlock hacks.