With modularity, everything gets much simpler if you centre your walls on the grid line (of whatever your grid size is, ie: 3x3m). They should be flush (no overlap) with each other when laid out in a line. You will need a piece to cover the gap in the outer corners, but the inner corners will overlap and take care of themselves. If you need to have the internal grid perfect the same guidelines apply, you just need a piece to cover internal corners as well. End of the day, you're either going to have overlapping somewhere (or a ton of extra pieces to cover all configurations), so take that overlap into account when you model your assets.
I think I went a bit too heavy handed on the fine details, which I've noticed seems to be a common theme and bad habit of mine that I need to watch for. Next week I'll be cutting back on that & smoothing shapes to regain the larger chitinous plates that were present last week.
That said, the finer details will be helpful in making the shapes of the larger plates more accurate to how the skin and plates articulate.
Yeah, the black outline are ray misses/artifacts. Basically means that your LP has too much of a discrepancy/gap to your high-res mesh. It's much more visible with a metallic and specular material, like what you have applied in marmoset. However, because a lot of the material would be polymer/matte, it wouldn't be as visible; especially once textures are applied.
I'd not worry too much about it. If you really want to solve it, just make sure your lp vertices are snug against your highpoly to reduce that issue.
you can disable color space manipulation when opening files in the color settings
I'd advise that if you're doing anything with normals or other 'maths' textures (roughness etc) as photoshop will apply (and save) transforms, you won't know about it and suddenly what you thought was a value of 127,127,127 will actually be 141,141,141
photoshop isn't doing anything objectively wrong apart from not warning you about it - its just not aimed at working on images that want to be manipulated in linear space .