15" wide? You have to be kidding!
Here is the bar width discussion
Seatpost size - can usually be found on the frame manufacturer's web site or use calipers to measure it. Your shop can probably eyeball it and give you a measurement from sheer experience.
Brakes - did you have them set up backward on your road bike for a reason? What ever you want is probably fine but don't ever loan your bike to some one else without HUGE caveats and warnings.
USE used to make an excellent suspension post (that being relative to other suspension posts).