Understand that where you break your hands dictates the path the throwing arm takes to get to the cocked position and the path (along with how fast you move your arm) dictates the timing of getting to the cocked position.
When you break your hands also affects the timing of getting to the cocked position.
Having said that, I almost never fiddle with the hand break or the throwing arm - I find the timing is next to impossible to get right because it's all relative and changing one thing affects the timing of another.
See 1st comment above.
See 2nd comment above. Also, there's always 2 ways to look at things. In this case, you could try to get to the cocked position later. Or, you could speed up other things such that the arm gets to the cocked position right on time.
I agree this is an issue affecting your velocity. But I think your posture (head and upper spine) tilts to the glove side which pulls the shoulders open early. It looks like you're trying (or you've been instructed) to throw over the top. Try keeping your head upright and let your arm slot drop. "Take care of posture and let arm slot happen" - Tom House.
In the .PDF that was posted, I see Chapman's front leg being straighter than your's but that doesn't necessarily mean firmer. Plenty of big league pitchers plant with a bent front leg and firm up in that bent position. It just takes strength which you appear to have.