I envy you- I have always LOVED racing, but the only racehorses I have ridden are off-the-track.
Probably whoever you are riding the horse for will be more than happy to help you. Are you talking about doing a morning work, like an exercise rider?
I can't answer all your questions, but I can help with the one about slowing down. Jockeys stand almost straight up when slowing the horse down at the end of a arce for the same reason the crouch down during a race. Wind resistance. If you want a horse to slow down very quickly, like they want to do once the race is over, standing up increases wind resistance, slowing down the horse. It isn't absolutely necessary, but it helps tremendously. When you want the horse to go faster, crouching down eliminates resistance, making it easier for the horse to run. If you want the horse to fly, then you want to be crouched down near their neck.
You can still give the horse the normal aid of pulling back on the reins to slow down as well as rise a little higher. However, most racehorses love to run, and once they are started, it's hard to stop them. Be ready forthis and be on your toes. They can take advantage of the slightest attention lapse.
Hope you have fun, let us know how it goes because I would be really interested in hearing. Where is the stable you are going to do this at?