I don't think you need to shorten your stirrups, I believe they are a good length. I think the problem you are feeling is the pinching with your knees. Concentrate more on your lower leg (calf) wrapping itself around the horse. Concentrate on using your whole leg.
Do a lot of two-point work in the Trot & Canter, until your legs feel like jello. Do it often, as well. And while your doing, concentrate on not pinching with your knee. Let your heel sink down and your leg relax and wrap around the horse. If you can't feel your lower leg (calf) on the horse, then you are pinching with the knee. You should be able to feel it.
Try a lot of no stirrup work at the trot as well. Alternating between Posting & Two-Point. It will help you to feel the difference between when you are pinching and when you aren't, and will then better help you to fix it once you know what the difference feels like. KWIM?
Fingers closed. That is a big thing. I do think you threw a lot of your upper body weight on his neck and are holding yourself there with your hands on his neck, but I think this is largely due to your knees pinching and throwing your leg off. I think once you gather a stronger more solid lower leg you will find you no longer have to use such upper body.
Your release is good, I find it is a good release for this size jump.
Your back has roached slightly, and your shoulders have slumped. While you are working in two-point, and on no stirrup work, think about rolling your shoulders back and sitting up very tall. It may even feel like you are leaning back slightly, but you aren't. You want a nice straight back, and shoulders open. This could also be a reason why your weight is thrown off in these pictures. Once you open up your chest and fix your lower leg, it will put you in a better position to stay solid.
:) Hope it helps!
...he's so kissable, huggable, loveable, unbelievable...