In the starting position, retract the shoulders slightly and flatten up your back a little more.
You're right about the unstable bottom position. You do a good job of turning the bar over quickly just make sure you have a good punch into the barbell for a solid lockout. It looks like your right elbow is a little soft when you hit the bottom of the squat. Pull those shoulder blades together and keep pushing up on the bar for a solid overhead position.