I get what she's saying, and I hope that Chemaine can help us with that. I think that Speedy will probably always struggle with being too low in the poll if he wants to lift his back. I think the canter photo just above shows that he's trying to sit and lift his withers, but to do it, he needs to go deep in the bridle.
The trot lengthening though is a fake out on his part (that photo is above). I got a bit of a lengthened stride in front, but he's clearly not sitting and pushing from behind. So ... maybe he can sit better in the canter than he can in the trot?