Update KB4343898 for Windows 8.1-x64 (Aug 2018 Rollup, incl. L1TF fix!) stuck with "Pending...

The Aug 2018 Update Rollup for Windows 8.1 64-bit KB4343898 is a most important one because of the L1TF CPU bugfix contained therein. Unfortunately, on my main production Windows 8.1-64 installation, this update gets stuck in a continuous "pending restart" state. I can't get it to install satisfactorily, no matter what I try.


Remark: KB4343898 installs properly in a Windows 8.1-64 virtual machine of mine, running within Oracle VirtualBox under Linux (both openSUSE Tumbleweed and Majaro) on the same machine as the malfunctioning one. And it installs properly on another PC of mine. But it fails on my main production machine!


Symptoms of malfunction of KB4343898:

• Downloading and installing work as expected before the required reboot. At this point, the Win 8.1 Updater GUI (Start Menu > "PC Settings" ("PC Einstellungen") > "Update/Recovery" ("Update/Wiederherstellung") reports the status "Pending restart" ("Ausstehender Neustart nach Installation am ..."). The Windows Control Panel ("Systemsteuerung") > Programs ("Programme") > "Programs and Features" ("Programme und Features") > "Show installed updates" ("Installierte Updates anzeigen") reports KB4343898 as installed already at this point in time.
• Rebooting as required seems to do the extected actions, prepare Windows and install updates. However, the Updater GUI keeps reporting KB4343898 as "Pending restart", it does so perpetually.
  
• From then on, my Windows continues to do the Windows preparations and updates installation at every restart or cold start of my computer. It does this either right away, or it does it after the next check for update availability (both an automatic check the next day or a manual check).
• In conclusion, this most important update is not installing properly, and it is a time-comsuming nuisance at every restart.

In order to try the remedies I could get hold of, I am working from a system backup, fully updated except for KB4343898. I restored this backup every time a remedy would fail, and started from scratch again. In summary, NONE of the remedies worked.

1. I took the guided tour (https://support.microsoft.com/de-de/help/10164/fix-windows-update-errors) all the way through:
   1. The troubleshooting tools claimed to have fixed some things, but to no avail.
   2. Trying to fix the update components by renaming the software distribution folders C:\Windows\SoftwareDistribution and C:\Windows\System32\catroot2 (after stopping the four services wuauserv, cryptSvc, bits, msiserver) didn't help. This made the update history go away, but the system kept to do "installing updates" at every restart.
   3. Trying to fix corrupt Windows files via "DISM.exe /Online /Cleanup-image /Restorehealth" didn't get anywhere, DISM stopped with Error 3017, asking for a reboot in order to roll its changes back. No success with that.
   4. "sfc /scannow" would not run at all, saying a system repair and reboot were pending. Yeah, sure...
2. I downloaded KB4343898's .msu file and installed it manually. The standalone installer reported "successful installation, please reboot", wherafter everything was back to the perpetual "pending restart" plus "error during installation" status.
3. I tried to get rid of the C:\Windows\WinSxS\pending.xml file. "takeown /f C:\Windows\WinSxS\pending.xml" was successful, but the ensuing "ren C:\Windows\WinSxS\pending.xml pending.old" would throw an "access denied" no matter what I would do (looked at security settings for this file and changed them to full access). No luck with this published method, access denied! Funny enough, I was allowed to text-edit pending.xml and delete its inner contents. However, this didn't help the perpetual "pending restart" either. At this point, the Updater GUI reported an error code 80073712 for KB4343898 in addition to the "pending restart" status.
4. I tried downloading / installing KB4343898 without my antivirus software, i.e. deactivating the AutoProtect of Norton Security. No success either, as expected.

Thus, I am down to the following questions:

1. Anybody out there who could offer other methods of remedy? Anybody who has succeeded in getting this update to install? In case of a successful remedy, how would I know for sure that this update has installed in full and properly?
   
2. I am URGENTLY asking Microsoft to get KB4343898 fixed, since it has the MANDATORY bugfix for the L1TF CPU flaw.

I am sparing you the rant. Just one thing though: I used to run two Windows 10 machines, until one of the fat upgrade rounds sent them both down the drain by causing perpetual BSOD's. So I ruefully returned to Windows 8.1, hoping for a smooth sailing (well, that worked for a bit more than half a year until now...). So I have not only lost the money for two Windows 10 licenses to Microsoft, but now I received the headaches and pains I am reporting here.