1. More forward seats are for racing ; laidback for touring/general, also of course they are easier to mono and jump with.
It's largely a question of what's in fashion really.
2. They should all have two brakes in fact. Personally I would suggest you get one with a coaster brake (foot brake), as these are easier to stop and you can do burnouts and that.
3. If the frame can handle jumps then the axle should be ok.
4. I would not recommend alloy hubs. If you run them with loose bearings and the bearing cage breaks up, it will chew through the hub.
5. V-brakes is a trademark of Shimano, they are the same as U-brakes. Calipers are not as good. Foot brakes are more suitable for BMXs (a little bit more weight though).