OK. I think I am beginning to see the scope of the problem. And of course, Microsoft is not being helpful. The official Microsoft answer is as follows:
"Kindly try to start your computer in Safe mode from the Windows Recovery Environment.
Please refer to the below link to start the computer in Safe mode.
http/windows.microsoft.com/en-in/windows-8/windows-startup-settings-safe-mode
Note: The steps for Windows 8.1 applies to Windows 10.
Once you have entered into the Safe mode then, open Command Prompt and enable Built in Administrator.
Follow the below steps to enable Built in Administrator:
Open Command Prompt from Windows Recovery Environment.
Copy and paste the below command and press Enter.
net user administrator /active:yes
3. Close the Command Prompt.
4. Try to login though Built in Administrator and convert the your user account to Administrator or you can create new user account."
Another Microsoft help desk agent suggests the following (and it looks promising):
"We have come around many users who have lost administrator rights on their Windows PCs and thus they were not able to manipulate their system or make changes to it. One common thing we found regarding the issue is that the users who have lost their administrator rights, have only a single account on their system, which was of course the administrator account. As a result of losing the admin rights, you may have issues in using Windows Apps, making new or modifying Windows configuration settings in Control Panel, using Snap-ins and several other problems on your system.
It thus becomes imperative for us to get the administrator privileges back anyhow if we want to run the system without road-blocks. But how do you get back these privileges since for every operation you have to carry out, requires the administrative rights itself, which we have lost unfortunately. While performing a Refresh or Reset will fix the problem, you may want to try out our suggestion first and see if it helps you.
Method 1
•Open an elevated command prompt. (Right click on start button and select command prompt admin)
•Type takeown /U %username% /F %USERPROFILE% /R /SKIPSL (You may also need the /P <password>
•Once that completes type cacls %USERPROFILE% /T /E /G %USERNAME%:F
This should force you to be the owner of everything in your profile and grant you full access.
If the issue still prissiest, try Method 2,
Method 2
Press WinKey+ Q, type user accountsand click on the result.
Then select your User Account > Click Manage another account.
In the following window, click Add a user account option.
Now we have to create a local account user. So after clicking Add an account, click Sign in without a Microsoft account (Not recommended), then clickLocal account. Fill the details on the screen so appeared for adding local account. Click Finish when done.
Press Windows Key + Q and type cmd, for the search results, click Command Prompt. Type following command and hit Enter key: shutdown /r /o
The previous step will result your system to boot into Safe Mode. After getting into Safe Mode, press Windows Key + Q, type user accounts, and pick the same from results so appear. You would have now two accounts on your system; first you’re issued admin account and second the local account which you’ve created in step 2. Select local account.
Now click Change the account type link in following window:
Moving on, change the account status from Standard to Administrator. Click Change Account Type.
In this way, the new local account we’ve created has been changed to Administrator. Now you can restart your machine and log into this account with administrator rights. Since you have administrative privileges now; so you can back up your documents from the old administrator account.
Finally, delete the old admin account and switch to Microsoft account to synchronize your settings with new administrative account. Your system will now act normally with full admin rights in your hand."
If all else fails, you could try a system reset. I'll keep looking for alternative solutions. Let me know if any of these work for you.