Hi!
The problem about your lighting not finish building seems to be the same as this problem I had myself: Very long (maybe infinite) lighting build times on empty or very small scenes
It’s basically a bug with very small scenes. So that’s why it works when you add a primitive from the engine. That primitive is probably bigger than your object, or the combination of that primitive and your object makes the scene big enough. So, unless you want to create a scene with only a small object, it shouldn’t be a problem.
About your lightmaps, I’m not sure. Your UVs seem ok. Make sure you created them in the second channel and that the model is actually reading the lightmaps uvs from channel 1. You can see and change that in the property Lightmap Coordinate Index in the static mesh editor. Also, when working with BSPs, you actually have to lower the resolution to increase the detail.
I hope this helps to find your issue. Greetings!