hmm…if the light is set to dynamic does it appear correctly? If so, then I would be looking at issues with the lightmap resolution and UV layout. Even though you said you tried 2048, I’d be curious to see the layout if there is a difference between the dynamic and baked version. If not a difference I would be looking at the mesh position and moving the light troubleshooting where the error may be occurring.
This thread should help a lot with explaining the modular seams that you’re seeing. A lot of this information will be more formalized soon on our Lighting Guide for the Wiki. It’s a big topic and needs to be explained well.
While that thread goes into everything behind it, if you understand lightmaps and using the grid for your lightmap resolution then all you should need to do is adjust the World Setting > Lightmass > Lighting Smoothness to ~0.6 and Lighting Quality at 2 or above.
If don’t have a post process in your scene make sure there is one. By default eye adaptation is on and will make it easier to see in darker areas. I always disable this by placing a post process and setting the Auto Exposure Min and Max values to 1 before setting up all my lighting.
After turning auto exposure (eye adaption) off you should be able to see the results of the bounce light much more easily. 
This is correct that you can only have four overlapping. One for each RGBA channel. My question for you specifically now would be is this a game or is this for ArchViz work? If it’s more for arch viz work and you don’t need character shadows then use fully static lighting. With point and spot lights they don’t have dynamic shadows for anything except any movable objects and characters. Everything else gets static lighting. Beyond that using smaller radius for your stationary to have only 4 overlapping at a time would be the best method.