Am I the only one who has their own internal dialog on these forums? Pretty funny. Anyway here's what I found because I have a curious mind and maybe it will help someone else.
My original intent was to use one project file as a sort of sound design canvas where I could build drum grooves and shape sounds and try different things out, then import those sounds into the main project. It all went really well actually but I always had in the back of my mind that I would eventually want Session Drummer in the main project, not just the raw audio, so I could have more control over the MIDI sections, fades, instrument mutes etc.
That moment came where I wanted Session Drummer and 4 audio files that I had custom created to be a part of the main project.
Dragging the entire project in created a mess as described in the OP. Dragging the other way left out too many effects and automation. So I then created a track template of the test project and dragged that in but of course it was only a template so the MIDI data and audio was missing.
So I THEN imported the project into project that now had the template - and it worked. Weird. I just tried dragging the project only and it crashed again and I'll probably lose that project file to corruption. (When it happened before that file could not be opened again). But having the track template there worked for some reason. Weird.
Is there a moral to this story? Probably. Maybe don't assume a 'feature' is actually going to work. Auto Save. Save as. Do all your work in one project file. (I'm probably the only one who doesn't - or didn't)