Lighting getting impacted while enabling sub-levels.

Hi Everyone. I have a project with nearly 30 sub-Levels under a persistent level. I have added all my lighting under the Persistent. Now the sublevels are not evenly split because of their size and design . I will not be able to able to build all levels under persistent at once since theproject is huge and the lighting build fails even with 60GB RAM. WHen the lighting is built for an individual level with the Persistent level lighting, everything looks fine.

Now from here I have 2 queries:
a) If I enable ( make visible) 2 or more levels that are built seperately, will the lighting invalidate because of the mesh overlap?
b) If i enable the maps one by one the lighting changes visually, example: as I keep on enabling levels one by one the shadows start getting smoother ( lose sharpness) and the lighting quality degrades with more maps being unhidden. Am I doing something wrong? I will attach few images for reference. I have also tried streaming them using Streaming volumes. How can I fix this? I have also made sure that the lightmass settings for each level is same. (The Directional Light is set to Sationary).