1. For your one handed backhand, on your take back I would not have the racquet face as open.
2. On the follow through open up your chest more.
From looking at your "grip" I can't really see because of you bulbous handle, but try and turn it over more. as in knuckles on the top bevel. I think that will fix number 1. Try to get more of your hand behind your handle in other words.
This is a video me hitting a one handed back hand, but I'm sure a video of Dimitrov would be a better example.