I can't watch the videos right now, but if you are having trouble following the motion trotting jumps can be your best friend. And I'm not just talking cross bars here - as high as your horse and you can trot/jump comfortably.
I've had to do that as a necessity for a while now bc I have been working with a mare that really launched herself at jumps. By taking her all the way back - first by walking her over 18 inch/2ft jumps, then by gradually building her up to trotting 2'9-3 ft, we really made a lot of progress. For her it was all about learning that she had the power to get over jumps without launching (she's super athletic) and it really made a difference.
And while I was training her, I was also really helping myself get a better feel for following her motion. When you trot over jumps, especially ones that are a little higher, you can really feel the rock-back and thrust of the horse preparing to and then leaving the ground. I'd always had a little trouble getting the feel and timing for trot jumps where you don't have that "rhythm" that allows you to see your spot. After months of trot jumps though, I am a lot more confident, both trotting and cantering, because I'm more in tune with the horse's motion. Plus, if my horse ever breaks into a trot on a 3 ft course right before a jump, well heck, we know we can trot that fence, no sweat!
As for getting tense, that is another thing I had to work on with my mare. She could feel you mentally preparing for a jump and that excited her even more. I was advised by a grand-prix trainer to work on slumping before a jump - just relaxing my whole upper body, letting my shoulders round, my seat relax and my uppper body come a little forward (the exact opposite of my childhood hunter training). That allowed me to be ready to get out of the saddle when the time came, without signaling the horse about the exciting events about to occur. It really made a difference, both in relaxing the horse and in enabling me to feel and move with my horse's motion. (Of course, this ride works on an honest horse that doesn't stop dirty - you sort of need the opposite approach on one that likes to stop and spin right in front of jumps or that needs a more upright ride to give it confidence.) Even when I have to take a hold, I was told that if I could relax for that last second before the release, it would really allow for a more dynamic jump and it really works.
So I second MIEventer's sentiments. Form should follow function and too often, it is the opposite in the horse world. A more solid lower leg will come from practice, patience, 2point and stirrupless work. The rest will fall into place as it needs to so long as you keep working at it and have a trainer to help you through the briar patch. So long as your overall foundation is solid, your feel is good, your ride is adjustable and your aids are clear, that is all that really matters. :)