Well, I never figured out exactly why I was getting a namespace issue. But I deleted my .cache, .ros, .rviz, and ws_moveit2 folders and went through the build process again and now things are working. I think maybe what happened what I tried to install Moveit following this page: https://moveit.ros.org/install-moveit2/source/
and then went on to the getting started tutorial: https://moveit.picknik.ai/main/doc/tutorials/quickstart_in_rviz/quickstart_in_rviz_tutorial.html
which basically has you reinstall moveit. That makes the most sense to me as far as why I might be getting a weird namespace issue. (I did end up deleting the ws_moveit2 folder before I even got to the namespace issue. Maybe just deleting the .cache folder would have been enough? Who knows.)
Note: still looks like I am getting a namespace collision warning, but the tutorial works fine now. So I guess namespace collision wasn't the root issue...?
Thanks again Chuck for taking some time to help me investigate.