Your question/problem sounds very interesting since you tried already a lot or in my opinion nearly everything possible.
Some ideas that might help:
Idea 1: Broken folder right inheritance
Did you check that the path down to the folder has system rights? I saw that you checked for the current folder, but all folders above need that as well. The easiest way to do so is checking if the system right for the folder is inherited from root folders.
Steps to check the inheritance:
Step 1:
Right click on the folder -> PropertiesProperties.
Click on AdvancedAdvanced.
Now make sure that the root folder (inherited from) also has system rights.
Idea 2: Ambiguous Pathpath/too long path
Check your path it might be either too long or has symbols that "crashes" the path. To make sure, move your folder to a different place and rename. I already had similar issues with Windows.
Idea 3: New User Profile
If all that didn't help, you might want to consider a new user profile. To check if this helps, just add an additional user to your PC, login as the new user and add your folder to the index. Now give Windows some time to index and check back later if it worked.*
Idea 4: Windows Reset
ItsIt's the last resort, if. If you are running Windows 10 you might want to consider resetting Windows. (Settings -> Recovery)*
* Please remember when doing the reset to not install any program and just test the indexing since defective programs could influence the indexing.