Nice sling design
It is better to sling when you can in doing things , ware pain in the shoulder hurt more in the great thing others do in slings by post and pictures , I am in AW 
I know that a flopping arm and hand, sucks and the pain in trying
well we endure in trying. I did look at the pictures of many slinging, there limitations
What I did see ware many need to sling there arm higher to put less stress in the space in the rotator area of the shoulder and arm. The flopping arm can be help by a elbow support,
Taking your arm out of a sling and strengthening your limitation will help over use issues latter in life to. The best operation I could or wish I had for my birth erbs would be getting the arm and shoulder together , I know this will happen for you, Sling your arm is good , sometimes
improving your limitation will help overuse issues to. I know this by my post I hope this helps??
Merry Christmas
Tom