The reason he has an upside down neck, possibly before, could have been a high neck carriage. Like my horse, he is genetically disposed to look like that. However, with work, it should improve, not get worse--and your horse is very obviously pulling against your hands.
A thing about muscles--this is how people look at your horse and can tell what you're doing, whether you're on camera or not. Genetics have a lot to do with how your horse looks, but steady work can change just about anything. My TB looked like yours--very angular--when I was putting him through incorrect work and false headsets:
But with steady correct work, long and low, and beginning lateral work, I got this:
Muscles tell us how we are doing, no matter WHAT we think
we're feeling when we ride.
Over tracking does not equal collection. Look at some of the worst-moving western pleasure horses with cripple lopes--they over track at the canter. (Disclaimer: I ride WP, but there are still some poorly trained horses out there! We've all seen em ;) ). Watch how far your horse moves his leg BACKWARDS--if the hip is turned out instead of tucked in, the horse's hocks will move past the tail. Collection is when the legs come under, and STAY under, and lift through the back. Your horse's back is much too tense to allow this as of yet.
Small circles and slow speeds do not make collection. Small circles are for well-collected horses because they are hard--if you ask a horse to work on a small circle without proper collection, he will not do it correctly, or fight terribly because of it.
He dumped you at the jump because he was much, much too close to it. He even goes to jump it and then decides against it.
Stretching is not just for warm ups and cool downs; they are for rewards and breaks. When the muscle is contracted (think a high head set), it needs to lengthen and stretch every so often to get out the lactate acid build up. What is a lot of stretching to you, may not be enough for Oscar--let HIM decide. Remember, horses are not stubborn or lazy or meanspirited--instead they are confused, sore, or unable to do what we ask. If you begin training him with that mindset, you won't ever have to fight with him. We're training athletes, not teaching dogs to sit. ;)
Here is my post on Long and Low
, and I hope you find some use of it. Would also love to see pictures of you guys and hear if it helps him at all!