I was not able to watch your video...I used to have the same problem. I landed too much on my heels, and I developed pretty bad heel bruises, even with super thick binding pads. I can say this...as you get better at the backroll, you will start doing more of a roll instead of more of a reverse cartwheel. To make it more of a roll, take a softer cut. If you are not getting enough air, you are throwing it too early. As you leave the wake, look over your front shoulder. When you get upside down, let go with your back hand and hold the handle as if you were sticking it in your pocket. Hold it close as well. As you come around for the landing, point your toes so the toeside edge of the board hits the water first while you are still rotating. This will stop your rotation and create a super soft landing.
Doing the backroll this method creates a much slower and loftier roll, and it also makes it VERY easy to grab or spin. A pro once told me that I was charging the wake too hard for all of my tricks. I scoffed at him...I mean what does he know? I hated it at first...it totally screwed up my riding, and I couln't even do it the old way way anymore. Well, he was right...charging the wake too hard only made me raley out my inverts, which causes me to land too much on my heels. You probably won't like it at first, and you'll probably hate me. Give it time, though. I will work better.
Since I couldn't see your roll, I am guessing. It sounds like the most probable solution, though.
Yes, the board should be going a bit behind you, until you take your back hand off the handle. The backroll is a roll (rail over rail), not a flip (tip over tail).
Hope I could help.