Are you able to edit the registry offline as outlined before? You could rename the whole branch HKLM\software\policies and create an empty policies branch.
what you can do, install GPMC on other member server or on client PC from RSAT and access group policy management console from there and from there open affected GPO and undone all settings you done one by one and then reboot the DC, it will solve your issue
Cannot access gpedit.msc
Can I edit the registry, and remove the entries blocking the access?
getting messages like:
MMC Could not create the snap-in
MMC cannot create the snap-in because of current user policies.