Sore after 50 serves is kind of worrisome. Be careful. If it doesn't feel nice and easy take a break and reassess. I guess that's what you're doing now. A couple things - leg drive and yadda yadda can wait. I think you should put down the racquet and just throw some tennis balls for a while. It looks to me like your throwing mechanics are off. They were probably off before you got injured, which is probably why you got injured.
I would just go out and throw some tennis balls around the court. Some high, some long, some into the ground in front of the net. Just work on a smooth, effortless throw. Once you get it all flowing you can work on getting some snap into the motion.
That should get you ready to fix your shoulder over shoulder cartwheel, which looks broken. Read this: http://tt.tennis-warehouse.com/showp...6&postcount=36
Once you've got that down then start throwing balls from your serving stance. This works for me when my motion gets ****ed up.
Finally, your toss isn't far enough into the court which will cause shoulder impingement. Check out the frame at impact. See how the line from right hip to right shoulder to right hand is almost vertical? Now go to the same frame of some good server that you admire. Since you use a platform stance it may as well be Fed. See how his right hip to right shoulder to right hand line is angled into the court? He's really throwing his racquet into the ball with good leverage, while you appear to be hacking up at it using just your shoulder.
You've really got to go back to square one, and if I were you I would start with throwing tennis balls. Hope that helps.