After much experimenting, I can say it’s fixed.
Evidently when in a BoxContainer, FlowContainer, or GridContainer… creating children of the Tree doesn’t properly … or ever… show up. And if I tried without one of these, I wasn’t able to make it look clean because control parents running their children’s lives down to the pixel.
But putting them in a SplitContainer (and a panel, but not sure if that is required) allowed the items to properly showup.
I’m going to close this, clean up the code and get it working how I want properly and then try to port it over to a plugin after that. Thanks for the help!