Sounds like a good one. I hope it works, because just rebounding vs flipping is different as I feel I can control the distance/landing a lot more with a rebound, for instance: on a large trampoline I tend to really set my back flips, but that usually makes it so I start and end in the same spot...
Bent legs on take-off/first phase is most likely lack of flexibility in hips/quads. But it happens especially if you buckle your knees forward upon take-off.
Handstand strength and shoulder flexibility mentioned by someone else, is a good point for the bent arms and closed shoulder thing.
I'm a bit stuck on learning new skills and bringing them to the floor. Trying new skills on the actual floor poses way too many risks - wrong technique, not getting enough height or landing wrongly with way higher risk of serious injury.
So, as most of us do, we learn new techniques on the...