Probably because 700c wheels won't fit with the slacker seat tube angles in the larger frame sizes, but they can stuff one in the 43cm frame. I'll bet it's a tight fit though. Also, maybe the Fuji track uses a shorter fork for the 43cm frame size, but BD uses the same fork throughout. I think you are correct that the WH is just a Fuji track frame, but BD kept the same wheels and fork throughout to keep the price down. Of course, for the difference in price between the Fuji or IRO and the WH, you can buy a pretty nice set of 650c wheels.

A mini velo will do... I recommend a Silgey Piccolo. With 20" wheels, it can go quite fast. I have a Schwinn Qualifier with Greenspeed Scorcher tires. You can set up as a fixed gear if you like.