-I think your hips going up first is a sign of them just going to the optimal pulling position for you rather than any particular weakness or form breakdown. The goal is to have your hips as high as possible while still getting your hips horizontally into the bar. So my take is that you set them just a little too low and they went to the optimal position.
-I didn't really see any rounding I think alot of that will just be in your head, what you are probably seeing is your lats and erectors as opposed to actual spinal flexion.
-For a good sumo your knees should lock out and then a slight bump from the hips locking out to complete the lift (credit to Arramzy for that advice as its something I had not been aware of myself)
-I would just suggest very aggressively pushing your hips through to finish the lift, something I learned painfully when I tore my hamstring last December on a lighter set by having some laxity in my hips.
I think you have a pretty good hang on this technique though all things considered. also how tall are you, Im 6'1" and also pull sumo so we might have similar leverages.