We have that also...30" cabs on each end. I don't know what kind of mirrors you're doing over each sink. But if they're coming in from the wall a little...say two inches on each side, that's bringing your mirror width to 26". You may also want the lights to come in a little also. Personally, I'm doing double sconces over our sinks. I like the look this gives better and there's plenty of light with the other lighting in our bathroom. Some images for you to peruse:

I think I understand you wall situation, but not sure what gaps you have with the granite -- along the sides? Did you tell them there would be tile on the sides? If not, they should have fit that in closely enough to just caulk it. I have a hutch-like cabinet arrangement in between two walls and that's what we did. No tile or side treatment at all. Each side of my kitchen has cabinets ending in a wall or tall cabinet. We took the splash up to the upper cabinets and ended it at the side -- nothing turning the corner. That would be my first preference. There are some photos of terraced tile sides here. I can't think of names of who has them though. I think that looks better with more contemporary looks, possibly arts & crafts and more modern, and with tiles that have some accents, color variation or other feature that helps with the visual movement. In a more traditional setting, if you have to carry the tile around on the sides, I would probably bring it around more evenly. You could consider taking it to the ceiling -- as if the entire alcove were tiled and you plopped these pieces inside. Take a photo or drawing of the elevation of that wall and make several copies. Draw it out several different ways and try coloring with pencils if you want to play with colors too. Look at them several different times and see what you think overall. Even inexact little pencil lines on a drawing will help you get the feel and will probably tell you which way you like it best in your setting.

I think it definitely would add value to your house because the next owner wouldn't have to go through what your going through now. I think you're right that people want more usable, functional space. Too many people think they want totally open spaces but they end up with what you have now with wasted and useless space. and in furnishing those purposeless areas, they end up with something that looks like a furniture store showroom. Boring! I agree with squaring it off and placing the French doors facing the foyer. An added benefit when finished, is you gain some wall space in your living room for a wide but not too deep statement cabinet that could hold a lot of the kids' toys and games. If you have frequent guests and need an extra bed, I like the idea of a fold-up couch or desk/bed set-up to give the room an extra function.

Then I would do a 2-bulb that takes 100-watt equivalents and put it on a dimmer. If you can have a central ceiling light you could reduce the wattage requirements of the sconce since it would be more for evening out the light. But I think that with a three-bulb you may wind up with weird effects from the bounce from the light closest to the wall.