About the chair seat; my guess is that since that is more of a stock seat saddle, from what I can see, it will be really , really hard for you to keep your leg under you. You might be able to bring it back, but once you get moving, it will want to move forward due to the place where the leathers and stirrup bar are situated.
All in all, you sit rather nicely and have a good lower leg. I dont see all that much looking downward , myself, and you have a nice upright upper body. Hands are getting a lot better!
Use Sammys desire to rush . YOu WANT lots of impulsion. I realize that rushing forward in a "falling on the forehand" freefall isn't what you want, but you work toward shaping that. If you try to slow him too much, you have nothing to shape. You work , little by little to get a bit of bend in his body first. He looks to be really stiff and has a neck that indicates a long time of moving in a defensive and braced position. I would advise you to work a lot on getting him to reach downward and forward to counteract the large lower neck muscle that he has developed.
Also, do more lateral bending on him. The more supple he is laterally, the softer he can be longitudinally.