I was thinking about your post again.
You mentioned you modified your Registry with no positive effect.
Assuming you modified the correct Hive or sub-key, and added the correct REG value, the reason it will not work is because Windows Defender has Tamper protection. To succeed in taking control of Defender using the Registry, you first need to disable the Tamper protection.
Disabling Tamper protection can be done in the Registry, but rather than play there, you can simply scroll down the same screen where you toggle off / on the Real-time protection and a little further down this page, you'll be able to toggle off the Tamper protection. With that turned off, you can then modify the Registry, then restart your computer for its affect to take place.
Note: Disabling Windows Defender in this way leaves you vulnerable to malware infections. So once you've done what it is you need to do with System Restore point, I'd advise removing the modification to Registry and restarting computer to allow Windows Defender to run.