White Foot, I'm not understanding why you would pick up only the left rein to get the head set. When you are showing you are riding one handed, therefore must pick up both reins, it doesn't make sence to me training only the left rein. If you could explain your reasoning that would be great.
Pro, I will try and answer your questions based on what I do. I have a 3yr old Morgan x that I ride western performance (WP, Horsemanship, trail, little bit of reining). Storm did tend to move faster then I needed for WP, he is very forward thinking so I know what you mean about needing to get your horse to slow down. What I found worked was to go for long rides, 3-5hr trail rides. When I'm out on my rides I do equil parts walk, jog, lope. If they start off going fast that's fine, after a few days of this they will learn to regulate their speed to enable themselves to handle the length of ride. Basically this means if I'm going to walk for a mile then I want to jog a mile and lope a mile. The key is to keep the time at any gate long enough that they want to slow down because they don't know when you are going to stop. Once the horse starts out rides going slow then you can add in days where you only go for 15min or so but still doing every gait. The key here is that they don't know how long you are going to be out so they will conserve their energy just to be on the safe side.
Also when you are riding these typse of rides it is good to go on a long rein, this way they get used to responding on the longer rein without you having to worry about checking them back.
As for the head set, in the AQHA rules it says that "the horse should carry his head and neck in a relaxed, natural position, with his poll level or slightly above the level of the withers." Basically they want the headset to be as natural as possible for the breed/confermation of the horse being ridden. That being said, at the local level most judges are not registerd with AQHA so are judging on the old way of thinking, which is long and low.
How I train my horses to have a head set is to pick up the reins so I can feel contact with the curb chain, I hold the contact steady until the horse drops their head, even if it is only a fraction. When they drop their head I release the pressure. I keep doing this until the horse stays in the possition I want without needing to correct them. The best thing to do is to see where they naturally hold their head and work towards getting them just a little bit lower then that. If they are consistantly working a little lower than natural then they will be pretty good when you show. I don't have any pics of a good WP headset unfortunatly.