"I ran regscanner and deleted all keys/data it found relayed to Avira, still the Feature update to Windows 10, version 1803 says Avira AntiVir cannot be removed"
--- That’s definitely a drawback.
Try the Troubleshooter in control panel for Windows Update.
It should create a report of what's wrong, maybe even include the location of Avira AntiVir or remnant of what cannot be removed.
I know I'm speculating but maybe whatever cannot be removed is considered a system file for whatever reason.
--- Consider selecting Show hidden files, folders and drives (I believe you may have already done that part) and also unchecking Hide protected operating system files [Recommended] in the View tab of Folder options.
--- Make sure to set them back to Restore defaults afterward.
What anti-virus protection are you using?
With all of what you have tried, the following ideas might not make sense, but they only take a couple of minutes to find out.
Try ccleaner http://www.ccleaner.com/
See if Avira is listed in the Applications tab.
Check the cookies: Select Options and then Cookies
--- if there are any Avira cookies in the right panel, move them to the left panel.
Analyze and run the cleaner.
--- Run the registry: look for any Avira entries.
Go to www.belarc.com
Upon completion, scroll down to the Virus Protection section
Scroll down to the Software Versions & Usage section
--- If Avira is in the list, you can Mouse over to see details, click to see where software is installed