I believe that the initial acceleration for the 1h backhand is the key for a high level backand. I think pro players get that by starting with their upper arm jammed on their chest and turning the upper body.
Yesterday, on Tennis Channel, Almagro hit a high pace backhand winner down the line. They showed nice camera views after the shot. I used the DVR to stop and view his backand from the view from behind. His upper arm was tight against his chest and his upper body turned a lot. See if you can find a video that shows this part of the backhand.
When I look at your backhand I see space between your upper arm and chest. Needs more and better videos.
According to my view, in your case, if you turn your upper body this space will decrease a little and not accelerate your upper arm as I think I'm seeing the upper arm accelerate in the Almagro video. Maybe Almagro's arm is also semi-straight when he starts the upper body turn.
This issue needs some better videos to see how tight your upper arm is. It also needs some more pro videos to establish that the upper arm is actually tight against the chest and also exactly when during the stroke it is tight.
Justine Henin. Other camera angles would be better for this issue.
You can also experiment by keeping the upper arm pinned on the chest when you start your 1hbh with upper body rotation.
Maybe someone who has a high level backhand can comment on this issue or take a video.