I would love to see the horse go in a plain snaffle. Because you are a bit of a hand dominant rider, the kimberwicke has created a lot of contact evasion issues in the horse. (ie what Tiny has pointed out - the hollowing and running)
I would also really like to see you with a shorter rein and far, far quieter hands. It is really evident in the spook how much you rely on your hands and not your body to ride. You threw yourself right forward onto the neck, raised the outside hand, pulled and took your legs off. Were you on your seat and riding from your body in the first place, the spook would not have happened, and any little jump or scoot forward would have been half halted through your body being strong.
Your hand position is good, now they need to become quiet. Press your hands into the neck, use a strap if necessary to keep you from being too active with the hand. Have a short enough rein to have a good contact and then ride to the bit. The horses "headset" as you call it, will come naturally once he trusts the riders hands and is ridden forward into a contact. Sit on your seat, press your stomach forward between your hands and press your shoulder blades down your back.
With the transitions, in general, the preparation is lacking. Once your position is in a better place and your hands are not doing all the riding, it will be much easier to communicate with subtle and accurate aids what you are wanting from the horse. Use your seat to ask him to wait, position your legs, apply the aid and then allow with your seat what you are asking for. Count 1-2-3 for your aid. The aids should not be quick, they should not be hard. Give the horse time to know what you want, set him up to do what you want (the half halt - asking him to wait with your seat) and then let him execute what you have asked.
Overall I think you are a good general rider - however, there is always room for improvement. Equitation goes far further than thumbs up, flat back and heels down. Now it is time for you to learn the subtle things which will really teach you to ride and train.
I started Maverick in a snaffle and he took off on me whenever I asked him for a canter and he'd get his head in a high position and would refuse to bring it down which is why I have him in a kimberwick now.
I re-watched the spook part a couple times and realized that in the video you can very evidently see me take my outside leg off Mav when I turned him around but the reason I took it off so much is because I gave him a big boot (so that he'd focus more on me than whatever spooked him) as I turned which you can't see because I turned him. And I'm not sure that my position caused the spook because he never saw the out doors until he was 5 so he was VERY fresh when we got him and spooks at EVERYTHING. Although he's come a LONG way, even lunging him in the arena at home he'll take off bucking sometimes when a tarp blows or he hears a funny sound. So I don't think I CAUSED it, but you're right, it's possible that it made him more nervous. I'm used to him doing that though so.
Thanks for the comment on where my hands are, that used to be a BIG problem that I just got over. My hands would be lying on the horse's neck or lower which brought me WAY forward in my seat. I also had a horrible chair seat which I've corrected for the most part but I sometimes find myself getting lazy and my legs will creep up again. I've been told that my hands are too far apart though, is that true from what you can see?
Also thanks for the tips on transitions, I've been watching it more lately.
Thanks a lot for the reply :)
You seem to be doing well! I agree that you ride too much on your hands. Try sitting deeper (heavier) in the seat, and raise your hands a bit. This should also help your horse's "headset", or get him into frame. You guys really looked great though! Keep it up!
Thank you :)