I finally figured out the fix to the issue. I had tried multiple registry edits suggested on many other forums. I had tried the System File Checker but that had always reported everything was ok. I had even re-installed AVG and used their removal tool. Still nothing got Defender re-enabled.
What actually ended up leading me to a fix was someone pointed me to an article where a user had uninstalled AVG and wasn't able to run Defender just like myself. For them the fix ended up being to remove SpyBot's active service. That seemed pretty reasonable but I didn't have SpyBot installed. It did get me to think of other programs that had a security focus though. I had Comodo's personal firewall installed. Comodo does offer other services such as AV in their suite of products but I was running just the firewall portion. Still, I figured it was worth a try. I uninstalled Comodo's software and that did the trick. I even re-installed the Comodo firewall and Defender is still running just fine.
So, the final fix was, uninstalling then reinstalling Comodo's firewall product.