Ok, I'm going to go by your 2nd video....the first was a little to wide shot for me to see anything definitively.
Ok - first, you need to get Sandie to respect your leg. You are really using alot of leg to get her moving, and you shouldn't have to. You need to work on getting her to respond to your leg when you touch her hair.
Dorothy Crowell worked with me on this the last time she was in my neck of the woods. At a stance, ask your horses hair. If you get no reponse, ask her skin. No reponse, ask her ribs - no response - kick her very hard. Like, legs far up in the air and down ask. She moves forward, stop her and repeat. It should get to the point where all you have to do is ask her hair.
Then, when you are working like you are in the video, you can have her infront of your leg, instead of behind it. She needs to beable to respond to your good leg, especially for when you are out on CC and doing bigger fences.
Back on topic - I'd like to see you shorten your leathers a smidge more, and you get out of the tack - for jumping. You need to work on a functional 3 point seat instead of being in a dressage seat. Dressage seat is fine for when you are doing Dressage, but not for when you are jumping.
I understand Sandie needs the support and the drive, but I think you are at the point now where you can adjust yourself while staying in that functional 3 point seat, while keeping Sandie infront of your leg and driving forward.
What you need to do, is work on standing up in your irons while doing walk,trot and canter work. Get your legs under you and learn to balance yourself while standing up in your leathers. And I mean, standing up. Find your balance, find Sandie's balance.
Now, while you are up in your leathers and after you've accomplished your balance by being able to walk, trot and canter in this position without aid from your horses neck or mane - now work on your "Breaking Over" position which is your jumping position.
What you do, instead of thinking "fold over" think of yourself as an accordian. Your knee's and your seat. You cannot think of it as your upper body, because it is not - your upper body doesn't move - it is all about your knee's and your seat.
So, back to the accordian, imagine your lower body as the folds in the accordian. They fold down.
So, back in your standing up position, moving into your jumping form - you close your knees, close your hips and push your seat back to the cantle. Hold it for a few strides, and then go back to standing up.
Now, merge from your standing up postion - to your functional 3 point position. Legs under you, balanced, activated core, open chest, lift your heart up, hands low. Feel Sandie inbetween your legs, don't allow your seat to rest in your tack, but keep it lightly over it. Hold this.
Now go from a functiona 3 point, to your jumping position. Remember - your body is an accordian. Now back to your functional 3 point position.
This will help solidify your position.
There are 5 phases to a jump, and believe it or not - the majority is done by your horse. Do me a favor, take your right hand, and hold it horizontally infront of you. Don't spread your fingers, and imagine your hand as the horse.
As you read the 5 phases, move your hand as though it is the horse -
1st phase - the approach. ( your hand should be like this - )
2nd phase - the take off (your hand should now be like this \ )
3rd pahse - over the fence (your hand should be like this - )
4th phase - the landing (your hand should be like this / )
5th phase - the depart (your hand should be like this - )
Now, take your index finger on your left hand, and place it to your right hand and imagine the finger is the rider. Keep your finger verticle from your right hand and don't move it.
Now, do the 5 phases again, moving your right hand.
Does it make sense? All you do, is just stay over Sandie's center of gravity by remaining in your functional 3 point position, and close your lower body as though it were an accordian. Sandie, does the rest - so by you solidifying your lower leg and your core, and solidifying your functional 3 point position, you will stay off of Sandie.
What you are doing - is hollowing out your lower back and standing up in your tack - instead of sinking down and low.
To help you with your release - well, you don't need help. I don't see an over release at all to be honest. Heck, it's far better than mine - because mine is non existant. lol.
But what you need to do, is activate your core, and lift your heart up. Instead of thinking shoulders back, think "lift my heart up". And I don't think your reins are too long, they are exactly where I ride. If that works for you and Sandie, I wouldn't worry about it.
Let me know if I made any sense......