WOW! You really have a problem. First, the weight issue: A Vet/friend put me onto Seminole Wellness Perform Safe. It is a pelleted, NSC feed. I had great success with it for my(at the time) 28 yr old TWH. Then two friends were having problems with their horses weight and I put their horses on it with much success and they have remained on it. Once you establish the perfect weight, then you maintain or slowly decrease amount to the perfect amount to maintain. The seasons will dictate the amount to increase or decrease once established.
What did your vet suggest for the pacing? My first reaction is to call my vet. My horse has cancer, so I am very careful at to what goes in and on him. Let me know what your vet says.