I didn't read through all the posts, but what your horse needs is exercise, not a calming supplement! It's surprising what some consistent exercise will accomplish.
Really, I think it's sad people feel they need to "calm" their horses, when what they really need to do is get them out and work them. Ride some hills, trot, canter, go on a trail ride for 5 hours. Those are the types of things that calm horses.
I've owned two Arabians and a fiesty Fox Trotter and I never lunge. I just hop and and RIDE. Don't hold them back, let them burn energy under YOUR terms. I do all my faster gaits on the way out (away from home) and ask them to walk coming home. If you burn enough energy they will learn to conserve it.
I think it was Clinton Anderson that said his horses are nice and calm because they don't know when he climbs on them if he is getting off at 7:30 AM or PM.
So they have learned to conserve energy.
It's like dog breeds. Some dogs are not good for apartments. Arabians are like that. They were bred to have endurance and a lot of go. It's what makes the breed excel as trail and endurance mounts. It makes me sad to think people want to "calm" them. Calm comes from mileage.
Riding riding and more riding will calm the horse.