Not to be rude or snarky, but if you have all the answers, as it seems from your post, why did you ask for help? Just curious. You KNOW the horse is not hurting, you KNOW the tack ALL fits perfectly, you KNOW you are both riding identically so the horse is in NO way confused.....so what are you looking for here? Just to know if the rein with a curb can be confusing? Look at the mechanism of how a curb works as opposed to a snaffle, and you may be able to answer that too.
As asking for the canter, I always ask with the opposite leg for the lead I want. If a rein is picked up, it's the rein on the direction of the lead to assist in lifting the shoulder to encourage a correct lead depart.
If the curb has a broken mouthpiece, you can pick up and isolate the shoulder the same as you would with a snaffle. The contact is quicker and should be lighter.