My contractor/electricians put my sconces too high and it looks so bad. By the time they told me about the installation the wiring was all done and not enough slack was left. My boxes are at about 70" now after I brought them in and down a little. Maybe that's OK for a downward facing sconce, but mine are fairly rectangular and mount flush to the wall. I would say 66" would be pretty good. The point of bathroom sconces is to light the sides of your face. For a guy it's great so you can see under your chin to shave. The tall sconces make my bathroom look like it's off scale.

Yes and consider 1-light sconces at left and right of left mirror and right mirror and a 2-light sconce in center.

With the sink being offset,I would use both for balance.One over the sink,one over the vanity area.As long as the frame isn't too decorative,your tile will still standout.

These are great questions! Drawn from our guide to light sizing: http://bit.ly/1cuh1Fj Allow 4 inches of space on either side of the mirror that you want to flank with sconces. That 4 inches provides the space for the sconce's electrical panel. Mount sconces 36 to 40" apart, flanking the mirror, 18" from the sink's center line. If the sconces have shades, put the bottom edges of the shades a little below eye level (60 to 68" from the floor). I hope this helps and let me know if you have more questions!