I havent watched the videos, but a lot of your comments are disturbing, pause, burn, straight legs, bouncing off chest, lower slowly etc
I never thought for one minute this exercise was that complicated, obviously I was wrong.
Bend over, make sure your back is flat, pull bar to belly button, repeat.
Your legs should never be straight, you should be using enough weight to make sure its impossible to pause, whether your 45 or 90 degree is irrelevant, the bar should be nowhere near your chest