It may just be me, but I always have my Dad reminding me to keep my shoulders back and my elbow up. It looks like you might have the same issues as me with hunching a bit and dropping your elbow in the swing.
Other than that, I don't know a ton about proper form and all that but I can tell you that when you get ready to move up to the live cow and horse, you'll definitely need a dang good roping horse to learn off of. Believe me, I know how hard it is to try to learn to rope the same time you are teaching your horse to track a cow and position up.
You can't focus on your swing and release because you are having to keep correcting your horse and you can't keep your horse in the right position because you are focusing on your swing/release and where the cow's at. There is very little in this world that can get frustrating as quickly as that can LOL.
Have you looked around for clinics and stuff in your area? Last I heard, JD Yates was doing some clinics and things around the country. Not much of a horseman but a **** good team roper. Someone who really knows the ins and outs of being a great roper can teach you a whole lot about your position, swing, timing, and release.
Here are a couple of sites that might have some worthwhile information for you. http://www.teamroper.com/ Team Roping Clinics - NRS Events - National Roper's Supply - Roping, Horsemanship, Barrel Racing, Horse Training Schools/Clinics