It's really hard to see very much because so much of the time you are far away from the camera.
AS you know, I think Squiggy is a super nice horse. She seemed a bit irritated there and kind of holding back in both the trot and the canter. That holding back had her a bit "bunched up" looking, which I think was making it harder for you to have as nice a posting rythm as you usually do (and you do!)
AS for getting her to relax by deepening your seat, do you mean that you took less weight into the stirrup ad put more into your seatbones? If so, then it might actually discourage her from stretching out more.
Sometimes when a green horse gets that kind of bunched up way of going, to help it stretch forward and lift it's back, the rider will ride in a kind of half seat (I am talking dressage here, mind you) and kind of roll themselves more onto their thighs. This takes weight off of the seatbone and puts it more foreward. It might be that it puts the horse more on the forehand, but if you also ask for more push and longer , freeer strides, it is ok for the horse to be a bit more on the forehand to help her open up , reach, relaxe and stretchforward.
She seems to be neck reining rather well. I think you've done just a terrific job.
Remember to look up and forward, helps horse to be focussed forward. Don't limit yourselves to the two or three feet in front of her nose.