are you redoing the countertop and sinks or just the lighting/cabinets? If you have a single handle faucet, you can fudge it to the side a bit and angle it, then line the medicine cabinets with the faucets. If your bathroom is really traditional this won't work but you can do it in a more contemporary space.

Has your designer shown you a drawing of this plan? You should be able to tell from that whether you like it or not. Sconces provide excellent light at the sides of your face and are wonderful to supplement indirect lighting from recessed fixtures in the ceiling. The mosaic accents sound very nice.

What are your reasons for only using only one? It is a stunning statement lamp - does it serve your function? You are aware of shadowing from above. Be advised there is shadowing from the sides as well. Is it a design no no? Probably. Both from a form and function standpoint.

Jamie, I dont agree with cpaul1's response. I a Roburn medicine chest is very stylish and very up to date. I like the kohler devonshire traditional style medicine chest. I have a medicine chest at home and my bathroom lacks storage so I find it very useful. For lighting, by electricial code you will probably need an exhaust fan and maybe a light. I would use recessed lights in the bathroom itself and add a wet location recessed light in the shower or above the tub. And then add 2 single sconces on the sides of the mirror. Match the finish of your bath hardware. Chrome bath hardware, chrome fixture. Typically Chrome is more modern and polished nickel is more traditional in lighting. bath hardware usually comes in both finishes in each style. Id suggest http://www.hinkleylighting.com/product/chrome-ashley-sconces which for a source of light on the sides you want either a 75W or 100W equiv in each sconce. Also add a dimmer to the sconces which helps to adjust to a comfortable level. it also extends the life of your bulbs!