Found a displacement edge abutting multiple other edges.
Warning: overflowed 14 displacement corner-neighbor lists.Finding lightmap sample positions...
Displacements are indeed the most important issue here.
You have some crazy shit going on there like:
lots of brushes where the whole brush instead of just one or two sides of it are turned into displacement,
micro brushes turned to displacements, angled brushes turned to displacement, overlapping displacements,
big size differences between width and height of displacements...
While all these are issues, i guess only the micro displacements and whole brushes turned to displacements that also touch each other are fatal.
The rest just doesn't work well for getting good visuals in the end.
There are a lot of other issues as well that decrease the vis compile times drastically.
You should focus on sorting out these issues first, before even thinking about the visual polish stuff like displacement snow or fog.