Don't know about any rules of thumb. Mine are maybe an inch or two higher than the mattress. I'm happy with them at that height and wouldn't want them to be any more than an inch lower than the mattress. I could live with them being maybe 3 or 4 inches higher than the mattress. I think the reason I want them to be a bit higher than the mattress is because with the pillows and comforter on the bed, the bed will look even taller and the table even shorter. And then the edge of the comforter seems like it would be more likely to knock stuff off the nightstand if the bed is too much taller than the table.

I'd hang that art some inches above the headboard. It does not looked centered in the photo, but that could just be the angle. (Just noticed this is supposed to be a temporary location before you truly hang it!) Bedside tables should be high enough guests can keep stuff on there, for easy reach during the night. I'd say higher than those, or at least more forward-reaching. Also, lights they can reach from their beds so they can read. I also like outlets in the area so if I am reading a Kindle, say, I can just reach down and re-charge. Without getting up. If the bedding is white, I would not want white curtains. I doubt a hospital feel is what you are after. With gray walls and white fabrics, that's the feel this will have.

There are two designs but you can turn them to get different look on one. These are much prettier in person. They also have very pretty mirrored plaques.

Yes. The issue isn’t merely about symmetry, though. It is about use. If you have a partner and go to sleep or get up at different times, it might be helpful to have two lights. If this isn’t an issue for you, then....one lamp can work!