First off, props for taking constructive criticism.
When it comes to looking less stiff overall, I find that actually relaxing and not over-thinking as you ride helps more than anything. Just "feel" it, and your riding will look more natural.
You are also stiff on your rail moves. If you stiffen up during jib tricks, you will not only look super awkward, but you're also more likely to fall. When you hit a box or rail the right way, you let your momentum work with you, and after a bit of experience, your muscle memory will take over and you'll look better as a result.
As for your spins in particular, try to wind your body up more before the actual spin. Then, just before you actually start spinning, make sure your body is "open" to the spin and turn your shoulders into the direction of the spin.
It's hard to explain it without visuals. Check out this climp from "Jumping with Jussi" for an idea of the exact mechanics of how to spin:
I feel like I get what you're saying about the rotation. I'm going to come back in a few weeks with a new video, and see if I have improved anything.
The basic problem is that your are hucking the flip with no conscious plan on actually landing. Whenever you do that (even on a straight air)... your body unconsciously automatically "opens up" (i.e. chest facing the downhill) - which is natural ... except for the fact that snowboarders need to land "sideways."
You should really practice doing backflips on a trampoline and learn to spot the landing and not freak out and try to twist your board to get the closer foot (often the wrong foot) under you. For a backflip, when you see the ground (when you are basically a little past halfway in the flip and completely inverted), you need to consciously pull your legs under you and get ready to land.
im no expert on flips but on the first one it looked like you were in better position to stomp it but you opened up your body and landed sideways. the second video it didnt look like you got enough pop to get your board back under you