Ok, just an update. I tried the system in another project I’m working on, and it works perfectly! Which is excellent. The levels load and the default UI works and everything, which is very cool. So it’s definitely not something wrong with the asset itself. 
But it’s really unfortunate that it doesn’t work in my current main project. I have a feeling it has something to do with the fact that I already had a root/persistent level setup, and that I was using world composition, or possibly both. Plus it’s a fairly large project with c++ code and settings that I can’t even remember everything that I’ve changed, which doesn’t help. The game mode I was using crashing the editor whenever I use “Start Players As Spectators” is very weird as well.
I had a backup of the project it’s not working in, so I’m going to retry installing the LSS system again, into a fresh copy and see if there’s some configuration I can use that will make it work somehow. One thought right now, is there some way to make my current persistent level (with the sub levels) NOT be a persistent level anymore. I guess, to put it another way, is there a way to un-sub-level the current sub levels? Just a thought.
Thanks for your patience. I’m glad I’ve narrowed it down to the problem just being with my one project for now.