Upgrade from Windows 8 to 8.1 failed with C0000022, unable to repair or rollback

Question

I attempted to upgrade my desktop PC from Windows 8 (Pro, x64) to Windows 8.1, using the "Update via the Windows Store" download method. The update failed, and now I can't use the PC. I cannot find any way to repair the Windows 8.1 installation;
nor can I find any way to roll back to the old (functional) Windows 8 installation. I've searched the web for answers, and chatted with MS Support staff, and still no resolution yet. Any assistance to regain a functional system would be greatly appreciated.
Details
The PC is a decent spec box. No longer high-spec, but still well above the minimum requirements:
6Gb DDR3-1333 RAM
Intel Core i5-750 CPU
256Gb SSD drive as the primary Windows system partition (C:) with plenty of free space
1Tb HDD as a second partition for User data (D:) with plenty of free space
DVD drive (E:)
NVIDIA GTX260 graphics card
Logitech USB keyboard
Logitech USB optical mouse
An older webcam & printer that required some manual driver updates to function on Win8 (everything else worked out-of-the-box).
The PC was running Windows 8 Pro (64-bit, clean install) without any issues, for at least four months prior to the upgrade. Before that, it was running Windows 7 (x64, probably Ultimate but I forget) again without any issues.
All Windows versions are legit MSDN installs (I am a subscriber). I've also seen other install issues in this forum related to ZoneAlarm - I have never installed ZoneAlarm so that is not a factor here.
The Win8 to Win8.1 Upgrade
A couple days prior to the upgrade, I enabled Client Hyper-V on the box. Did all the restarts & everything, and it looked like it worked successfully. Windows 8 still functioned as expected afterwards. I never got around to setting up an actual VM though.
Probably a red herring; just mentioning it in case it could be relevant.
Immediately before doing the upgrade, I manually snapped a System Restore point.
Installing the upgrade seemed to take an incredibly long time. I think it is supposed to restart a couple of times... it was maybe six hours or so between the first and second restart.
After the second restart, it flashed an error message for a split second. I was able to catch the message (on a subsequent restart) by filming the monitor & freeze-framing. The message is "Fatal error C0000022 applying update operation 2 of 758
(amd64_ava...)". I'm curious why this is dying on and amd64 file, for an Intel chip?
After flashing that message, it proceeds to a screen where it says "Getting Windows ready. Don't turn off your computer". I left it on that screen for at least 12 hours, but it never proceeded or changed. Strong suspicion it has locked up at this
point.
Subsequent restarts just result in it locking up at the same point.
Loading the WinRE from the original Windows 8 install disc does not help. The snapped restore point can't be found; and the "Startup Repair" option fails with the message ""Startup Repair couldn't repair your PC". The logfile SrtTrail.txt
states "Root cause found: The operating system version is incompatible with Startup Repair" which seems to indicate the system now thinks it is 8.1, not 8.
Loading the WinRE from a Windows 8.1 install disc (MSDN) does not help. The snapped restore point can't be found; and the "Startup Repair" option fails with the message ""Startup Repair couldn't repair your PC". The logfile SrtTrail.txt
states "Root cause found: Boot status indicates that the OS booted successfully" which doesn't seem like a helpful error message.
How can I get a functional OS back (either finish the upgrade, or roll it back)? What additional diagnostic steps could I perform that would assist in finding & fixing the root cause here?
A minor version bump of an OS should not brick a functional system! I would prefer not to do a full clean install as that would mean days of reinstalling & reconfiguring all my other (non-app store) apps, settings, etc... but if there is no other option,
it is a last resort. Of course if it comes to that, I may as well just try Linux instead...

Answer 1

OK, so Microsoft support were eventually able to resolve this. Details:
Apparently a few other tools are also available in WinRE (including RegEdit). This can be useful for connecting to a hive from the previous O/S, to inspect keys etc.
Windows 8.1 really, REALLY does not want to let you boot into Safe Mode, especially if it thinks you are part-way through an install. Usual boot-time key combinations would not work. Options from install disks would not work. The only way we could make
this happen was (on tech support's explicit instructions) to
physically yank the power cable out of the box, while it is booting, three times in a row. Apparently that is the magic combination to fool the system into loading safe mode.
Once in Safe mode, tech support went through the process of disabling/enabling devices and services one at a time, to identify the root cause: the NVidia driver. The version I had installed was from October 2013... foolishly I had assumed it would support
Windows 8.1 because the driver was only 2 months old, but apparently not. Anyway, updating the driver to the latest version resolved the issue, and (eventually) allowed the install to complete & get a functional system back.
I'm recording all the details here, in the hope that it may help someone else (particularly if you are not fortunate enough to have access to Tech Support).
Also, if any Microsoft staff are reading this, I would like to make the following points:
Yank the power cable out during the boot, three times in a row, just to access Safe Mode?
Are you freakin kidding me? It is 2014 and this is your latest, greatest flagship product. I should not have to risk physical damage to the power supply or PC itself JUST TO ACCESS SAFE MODE. Yes, you should be embarrassed.
Something I mentioned earlier that I want to revisit - a minor OS version upgrade should not brick a user's system. If there is a chance of that happening, you should take reasonable steps to assist the user in preventing that from happening. Using
this example, the first step in the upgrade process could've checked the computer for all the installed drivers & their versions, and then compared them against a list of known drivers & versions for compatibility, warning the user where appropriate
before proceeding with the install ("your xxx driver is known to be incompatible with Win8.1, upgrade it then try again" if it's on the blacklist; maybe "your xxx driver is not known to be compatible with Win8.1,
proceed at your own risk" if it is not on the whitelist). I accept that you cannot hope to cover every possible version of every possible driver, and that it is (theoretically) not your job to ensure other folks software is compatible with the OS. However
you could at least make the effort to blacklist/whitelist the obvious ones... with graphics cards you really only have a choice of NVidia, ATI and Intel. Helping users to avoid bricking their systems will at least make your software seem less user-hostile.