OK - I found the answer from JulieGoodnight on this subject - thanks everyone.
There are many excellent questions that you pose and I applaud your efforts in trying to get things right for the safety of your riders and for the welfare of the horses you influence. First, let me address the use of the curb strap with the snaffle bit. You are right that the curb strap will only come into play on a leverage bit; on the snaffle (direct pressure bit) the curb strap will never engage. This does tends to break down along English and Western lines because English riders think it is absurd to have a curb strap on a snaffle because it seemingly serves no purpose on the snaffle, since there is no 'curbing' action.
From the Western point of view, the snaffle is a tool that is only used on young, green horses; once the horses mature and are more 'finished' in its training you use a curb bit (which is better for one-handed riding).Those finished horses are referred to as 'bridle horses,' because they are well enough trained to ride one-handed in a full bridle, with very little pressure actually used on the bit. This is the end result in training the Western horses while some English horses will remain in the snaffle forever.
If your use of the snaffle is strictly for colts, you know that you'll be occasionally using a lot of pull to guide the horse in a certain direction and the curb strap is there to help hold the bit in the center of the horse's mouth. You should not need this advantage on a well trained horse, which is the perspective most English people have since they are not oriented toward colt-starting.
However, a beginner rider may also take hold of a horse's mouth and put more pressure on the bit than is necessary, so the curb strap might help in that instance, too. Even though the horse is theoretically trained enough that it shouldn't require that much pull, the rider is not discriminating enough to give subtle cues. So the purpose of the curb strap on the snaffle is to help balance the bit in the horse's mouth, regardless of why you need that balance. You shouldn't use a curb chain for this purpose; it just adds unnecessary weight and noise; the chain (as opposed to the strap) does not increase the pressure on the chin with the snaffle as it does in the curb bit. If your purpose is just to keep the bit centered, you should either use a leather curb strap or just a cord to connect the two rings of the bit.
You are totally correct that a snaffle, in many instances, is a safer bit than a curb and if someone is having a training issue with a horse, going to a harsher bit is rarely the solution and will usually make things worse. I spend a lot more time trying to get riders to go to a milder bit than the other way around. For more information on this, please visit the 'library' section of my website, www.JulieGoodnight.com
Whether the horse or the rider is lacking in training (sadly it is often both), if you use a curb strap on the snaffle, it should be attached to the rings of the bit above the reins, like on any bit for any purpose. Remember its only purpose is to align the bit in the horse's mouth, so its adjustment should be with no tension between the rings of the bit, but it should come into play if one side of the bit is pulled out of normal position. The adjustment is without too much tension, but without too much slack.
Ironically, most people use the equipment they use because that's what everyone else uses; they have no idea what they are doing, let alone why. It is important for us as instructors to know why and to question why they are doing things. It is also important for us to be able to answer students when they ask why they should do something we told them to do. I hope this helps you realize that you were on the right track and asking the right questions.