You don't have to know what will go on the upper part of the wall. Prep and prime it and you'll be ready when you decide. I think you can use that medicine cabinet if it is something you love. I'm not sure how to judge your "is it too much" thinking though -- do you really love it? As far as size, make sure you have the height for it with lighting that you like. Include the height of your vanity, the splash, some room above that, the mirror and then see what you have left for lighting and clearance above and below. I would like the wider mirror, but if your ceilings are low, you may need to go shorter. If the mirror fits, what would you think about getting some rub n silver from the craft store and adding some silver highlights to the mirror and lightening it up a bit? If you like that idea, go to Hobby Lobby or Michael's and get some of that rub on silver (yep -- use your finger even), and give it some highlights. That might soften it a bit and tie it in to your lighter room. If you've never done anything like that, find something to practice on -- a picture frame, a flower pot or vase - something that you can play with and see how it works on edges and details. For lighting. look at the fourth row of that Ferguson's lighting link, look at the Feiss Claridge. That could follow the shape of the top of the mirror and it might be nice to bring in another curve rather than a straight line. Personally, I like a three light fixture most of the time. It's that odd number thing, plus, if that is the main source or only source of light in the bathroom, I think it's better to have the ability to have more light and tone it down with a dimmer or smaller bulbs if you need to or want to. As far as the toile, it's hard to say not seeing the space and what you have. It might look great, but you keep your other elements in mind. Glad you kept the grout light. I do think that the floral idea will work better than some other toile patterns because of the round and flowing lines. The ones that are tiny sketches scattered could wind up looking like one more liner element. All I can tell you about the creaminess is that I can't tell you anything from my monitor. You'll have to look at the different elements together. You should be able to order a sample of the wallpaper and you might want to try a few to compare.

Spottythecat, the towel bar is Pottery Barn 24" Mercer in chrome. kelwit, I used bullnose in the niche and Schluter for the shower wall edges. If you use the same shower tile I did, then Schluter, bullnose, or even the factory cut sides of the tile are all decent options. I used the factory cut edge for my bench seat and it doesn't really look too different from the bullnose honestly, plus it allowed me to avoid extra grout lines. Here are some closeups of the Schluter trim, niche, and bench for comparison.

Let's consider that mirror gone. In my heart I always felt it didn't work. You have confirmed that. To DLM's point, let me share the layout. The back wall of the shower isn't visible in the bathroom at large. The shower will have a curtain, not a glass door. I view the mosaic wall as a hidden treat for DH and I. I am still tinkering with actual colors. The red will probably be more brick, or rusty. The wall color will not be a stark white. It will play with the undertones in the red(hopefully). Here is a revised mockup showing 2 different wall colors, SW Glimmer(left), and SW Nonchalant White(right), and 2 different reds. I added an area rug and, for fun, a red mirror.

IMO a lot of backlit mirror and honestly not a good source of light for makeup inless all LEDs in daylight bulbs so 4000K and not more since then you will get a blue cast happening. I sure hate walking through the bathroom to get to the closet . You will still need some overhead lighting to balance the back lits and give good all over light .