I think you should look into polygonal modeling in a package like blender. I think it will give you greater control of how your asset turns out in-engine. When modeling directly with polygons nothing has to be “created solid.” A wall can be made (and often is) as a single plane right from the start. Also yes you can get away with having a single UV layout for your lightmap, however; that would lead to inefficient use of texture space. You want extra spacing between UV islands to prevent shadow bleeding in the in the lightmap. This is because there is normally a massive resolution discrepancy between the textures and the lightmap; lightmaps are typically a much lower resolution image so your UV islands need more empty space between each other. Overlapping UV’s is normally allow separate UV islands to use the same texture data. This is an intentional technique and should not be confused with issues caused by strange badly unwrapped meshes where a single island is overlapping itself.
P.S. that DNA ring is awesome.
Edit: This is a good place to start if you want to learn blender and get into poly modeling. I feel like Andrew Price is a good resource even if his focus isn’t in games particularly (generally creates finished renders). You can learn some of the basics about subdivision modeling and texturing from someone I find informative, friendly, and entertaining.
I’m going to attach some pictures of a project I am currently working on in Blender and if you have any questions in particular about its creation I will do my best to give you answers. 
The images are of a high poly subdivision surface model. When it is complete, I will create a low poly version and the details and surface normals will be projected from the high poly model to the low poly.
-Josh
