First of all, don't worry too much about "correct" head/neck position or "good" flexion, etc.. Now that he is coming into and taking a contact you have to worry MORE about if he is even in the contact, and what position putting his neck in helps you better access the body and the back. Again, what's in front of the withers will come, don't frett about it right now.
As far as YOUR position for helping him to take the contact better and better - your hands need to come closer together (and in some places come down) to "frame" the neck with the reins (I mean this in that the reins need to always be touching the neck). Having the reins in a constant position beside the neck is going to help him become steadier and give him more "contact" so to speak - but without pulling back on the reins.
Then you need to focus a lot LESS on pulling your shoulders back and a lot MORE on pushing your elbows and shoulders DOWN, and into your hips. Your elbows should be touching your hips at all times (and still be free to open and close). Having your elbows resting on your hips will spot you from pulling (pictures 1, 2, 3 and almost 5) and will encourage you to ride to the contact to strengthen it. AND it will make your arms steadier and more "inviting" to him to come into the contact.
Otherwise I see some good stuff, try to sit up a bit taller (which is hard on such a horse, I know) but it will help you ride to the contact better, and again discourage pulling. Good luck!
They say money doesn't buy happiness -- well happiness doesn't buy horses!