The white tile looks a little gray(ish), is that true? Because the mosaic looks as if the white in it is brighter? If that is the case, I would look for a different white tile. It may just be the photo. Either way, I would keep the shower just the white subway tiles with no trim. (I assume the tiles in the second photo are for the shower floor only?) There are shower threshold pieces that are like a Corian in solid white are available most tile stores & home centers. If the whites match I would choose the simple piece.

I like borders. I realize many people don't and maybe I'm not on trend because plain is the latest trend. I like a little pop of something with white subway. So I like your idea of the border. I think it is probably too busy to do the curb with that quartz pattern against that floor pattern. You could do a quartz white one.

With a busy patterned floor, I think the border is just too much.

I think some accent in an otherwise white shower is good, but maybe consider using a stripe of your floor tile? You could do a vertical stripe of the mosaic as shown, or you could pick out some tiles from a couple of mosaics to make a double line, say one dark and one gray, and then put them however you want -- vertical, horizontal, or just around a niche.

Otherwise, I think you'd be ok with the dark mosaic you chose, as long as it's a reasonable match to the gray in the floor -- would just use a bit to break up the white, though, like maybe in the back of your niche.

I honestly don't understand where the slate herringbone is going -- is that in the shower or on the bathroom floor? if the latter, then forget what I said about the shower; I agree that the floor mosaic will be enough, and you could maybe use a piece of it as a vertical accent stripe or part of the pattern in a niche. Or you could continue the floor mosaic up onto the wall with the showerhead.

As for the quartz, that seems a bit much next to that latter mosaic, no matter where it is, especially since one has pattern within the material and the other is a pattern made out of pieces. I think I'd just have the curb made out of your white tiles with white grout. Quartz would look great on a counter though!