Just cut a piece of 1x2" board about 12" long. Cut an "L" shape on the end that will ride over the thicker tile, which you put up first. Scoop in some thinset into the "band" area with a margin trowel and "adjust" or screed it with the board. Let it dry. NOW, you have a slightly raised area upon which to tile the glass and it will be the same height as the field tile.

Either the use of a thicker bed of thisnet or by using something to bring the mosaics out a little further. That could be something like Ditra, which would bring it out about an 1/8", or if it doesn't need to come out that far, they could even use strips of Kerdi or some other membrane to build it out in a uniform manner. No self respecting pro would tell you sorry, that's the way it goes! If they can't, or won't do it right, I'd hand them their walking papers.

Monrovia mom, I have a feeling you may be overthinking this. I have to preface this by saying this should be done to YOUR taste, not mine. But I'm thinking you may be disappointed afterward, doing what you're proposing. One stripe at say eye level, and maybe even a second at knee level if you really want to see a lot of the glass. But to have a stripe going horizontal, and another going vertical, and still more in the niche-- may become so busy as to be distracting. If you really want to do a lot of the glass, try something like this: As for doing the niche, and it showing as being off center, I think once you've had it there for a while, you won't even notice it, unless you're someone who is extremely detail oriented (read that as OCD :-)) Seriously-- overthink this, and it can drive you absolutely nuts. As for the water solubility of the glue, almost ALL glass is mounted with glue that will soften with water. Makes it a little touch on the installer, but not even close to impossible to work with. Once it's set, though, the thinset will hold it in place in spite of the mounting glue.

Those are custom tile work and I don't believe you would be able to purchase outright. But if you google "Fleur de lis tile mosaic" you should be able to find something to your liking. Fleur de Lis is a nice one. This one is more custom look and priced accordingly