AT the end of the last wtc video going left, when Ollie came out of the canter, he wanted to stretch down and that would have been a good time to do a "stretchy circle" with him.
He need to do more work with his neck both elevated (which he seems to want to do naturally), and elongated, reaching forward. At present, it looks like he is developing a way of holding his neck that makes it look scrunched back on itself. I mean, rather , the head is scrunched back into the neck, and it's blocking him.
He appears to have trouble moving his head to the right in the atlas/axis joint, right behind the poll. the movement that he would make if he was saying "no". When you were cantering to the right, and you asked him to flex and soften on the inside rein, he either raised or lowered his head to avoid that movement. I think he is blocked there. Stiff there, I guess.
At one point, in one of the videos, I saw you flex him to the left (his good side) and you did a pretty nice job of it. His head stayed in a vertical plane, which is good and correct. This needs to be done to the right, too, but with care because he demonstrates that it's difficult.
For you, when you trot, you are actually swiveling up, advancing your inside hip at the top of your posting trot motion, which is exactly when the horse has advacned his OUTSIDE him, so you are effectively counter swiveling his motion. You do this more going right than going left.
I think that anchoring your outside elbow, with some bend in it, to your outside ribcage will help. Both elbows need more bend so that your hands can stay still at the trot, but you really need your outside elbow firmly back, it is advancing in front of the inside one and is very stiff and straight.
At that canter you ride much better and could teach me a thing or two, as I am having some real difficulties there. Let Ollie stretch down sometimes, even at canter, and keep that outside elbow connected to your core!
With regard to the elevated/elongated comment;
think of making his poll go farther away from you. You want his face to be just in front of the vertical, but you want his poll to be pushing away from you, too. If you constrict him, he will raise his head but actually bring his poll backward, closer to you. just use that visual thinking; "I push his poll forward away from me"