Dark seams and light shining through opaque meshes

Ok, so I thought I’d try my hand at modular level design, as I need a large space for a current project, it seemed like the right way to go.

I’m currently creating simple meshes with very little detail in order to try stuff out. After a week or two of experimentation and much internet research, this is where I’m at:

I have two problems:

A: Why have I got that thick band of light above the window? The only thing above the window is solid, opaque mesh. The mesh isn’t sealed; it’s two seperate faces, like planes but I don’t see that that could be causing the problem; I have other meshes built the same that don’t let the light through.

B: almost all meshes are edged in a grey shadow. This is most obvious on the tiles, probably because they are smaller and more uniform.

My settings are as follows:
Static Lighting Level Scale: 0.1
Indirect Lighting Quality: 2 (but I’ve tried as high as 4)
Indirect Lighting Smoothness: 0.6
All other settings are standard.

I have a lightmass importance volume.

All of my models are built in 3Ds Max, have custom lightmap channels that have been snapped to a pixel grid with 2 pixel padding.

Lightmap resolution is currently set to 512 (but I’ve tried everything between 16 and 512.)

At this stage I’m totally lost. I’d really appreciate any help.