Great work so far duff! Love the commentary you do with the vids
With the back levers, you'll always have some sort of curve in your back when doing the tucked versions, so don't worry too much about it.
Practice doing skin-the-cat moves do build up the strength and flexibility in your shoulders. Also try hanging vertical upside down (really lock out your back here) and slowly relax your shoulders so you lower yourself down.
You really have to focus on keeping your back tight! (and don't worry when you only get to 30* before your back gives in - it's all building up the strength in your back!)