I'm definitely not a trainer, but overall, I like your leg and seat in the last pictures you posted. I think your trainer is right, it looks like you need a bit more height in the saddle (your horse is really uphill, lol), but overall, I think your leg is much better.
Compare your pictures to the picture that bsms posted, of good position according to George Morris- pretty darn close, if you ask me.
I haven't read all the posts so someone may have already mentioned this, but you may find it easier to keep your leg back if you lengthen your stirrups a hole or two and move your bum further forward in the saddle.
When sitting in the saddle, you should feel as if you are sitting on three 'points', making a triangle. If your bum is to the back of the saddle and you are slouching, you will lose the three points. To regain them, shift further forward in the saddle (in photo two on page one, you are quite far back in the saddle and you should sit roughly in the middle), relax your heels down and 'grow' taller through your pelvis, engaging your stomach muscles and lengthening your back.
Hope this helps.
By the way, you have a gorgeous horse!!
I don't jump. However, it is pretty obvious jumpers don't have their heels stuck back under their hips. This picture of Gen Patton helps show it:
His center of gravity is over the stirrups, but his butt isn't!
Littauer said riding isn't about how you look in a static picture, but how you move and balance on a moving horse. How do you feel about your balance? Is it with the horse, or behind? How is your balance affecting your horse's balance? Those are the things I would suggest discussing with your coach.
I'm a mostly western rider, but I'd suggest not worrying about a 'chair seat' and focus instead on how your balance feels. Talk it over with your coach and maybe she can have you try a few different things while in motion, so you can feel how the changes affect your balance.
It's the balance is over the feet. As one folds over the jump, the shoulders come down (of course in front of the feet) and the butt goes back to compensate. Otherwise, one is "ahead" of the horse or one's center of gravity is in front of the horse's.
Conversely, the infamous "chair seat" would be when the rider's center of gravity is behind his/her feet. Think about the alignment when one sits in a chair. Where is the "sitter's" center of gravity?
In riding the rider's center of gravity should be directly over one's feet. Or as my instructor says, if horse were pulled out from under the rider, the rider would land balanced on his/her feet.
For jumping (as my instructor also says), the crouch a baseball player or tennis player assumes "at the ready" is really the fold for jumping. It is the most balanced position for a person on two feet. The center of gravity is as low as it goes.
ETA: While the "vertical pole" through the shoulders, hips and ankles works when the rider is fairly straight, it changes as the rider crouches into a two point. I'm always told the same angle that occurs with the leg should be repeated at the waist. So the pole really will no longer pass through the hip and shoulders as those two weights go in different directions as the body is compacted.
Thank you everyone for your comments, I've gone back and looked at some of my other photos and I think what I'm thinking of that straight line, is more a dressage ride then mine. And I totally understand the weight needing to be in your feet more then worrying about the straight line - sometimes I know I over think things. I will continue to ride without stirrups as part of my schooling rides, which should help AND far more schooling two point, as I'm pretty week there. I find I'm either too low and not in a true two point or I'm way up too high.
But here is another shot. It kinda looks like my seat is in the saddle but I do remember that jump and it was just hovering above - it was probably my best landing of the night.