Hi Jeremy, I'm not an expert either, but I can tell you what I've found from trial and error works for me.

I would say you need to get rid of the back rounding ASAP, it was the first thing I noticed. Not sure if it's a set up thing back thing or an ankle thing etc. I am not sure if you already do this, but try to do overhead squats.  The bottom position is really important to get. I also say this because it could help with your back rounding.  You won't be able to OHSQ something if your back is rounded like that. It will also help with the bar position behind the head. I would say start light and worry more about hitting those positions with good form. I am also a huge fan of rows for back strength. When you started the set up your back was in good position, it was just on the floor where you lose it. 
I am not sure if you feel tight with your set up, but I find if I don't get a good set up and feel don't feel the tightness in my back and legs before I start, I am dead before I even get out of the gate. I usually grip the bar sort of in an RDL position and pull the slack out a bit, take a deep breath, then set my legs in. If in your set up you were trying to feel the position down to the floor I'd say pick the bar up get tight at the top and go down with it pausing then back up. With working weight pauses I just go knees floor knees, but this is just what works for me. Not sure if your back position will improve with this, but might be worth a shot?

In terms of missing the paused positions, I would say drop the weight and really feel the positions in good posture. It's sort of hard to tell from the angle of the video etc but it almost looked like you were just muscling it up? I am not sure if this is due to your back position being rounded? I am not sure what you consider the 3rd pull, do you mean the drop under? I would say focus more on keeping the bar close and finsihing the 2nd pull. There is no way to go under the bar if it's out in front that far. For me I would say my second pull is the make it or break it point with my weaknesses, I always think hips thru the bar to maximize power. If you don't finish that pull right you aren't going to make the lift like Nicholas said. I would say to really work on hips thru but make sure you aren't already on the front of your foot.

Do you have someone there who knows you well enough to coach you?  It's not easy saying xyz is the weakness from a video, but if you have a coach that can see all of your exercises, posture, form etc, it is easier for them to give you the best advice. I'm sure Pete or someone who knows more can give you some better advice too.

looks like your hips are banging the bar too forward. squeeze your lats hard to keep the bar from bouncing away from you. row the bar into your hips with your lats at the same time you extend your legs and hips. too much hips and not enough leg extension will bang the bar forward. keep the weight on the heels throughout the second pull. My coach tells me to "let the weight take you back. " for that split second that your feet leave the ground from pulling to receiving you should feel the weight take you back. This cue helps me get my hips through but it did take me a few practices to actually understand this cue and how it works. You shouldn't try to purposely jump backward but it should happen as a result of getting the hips through and really finishing that second pull. Also some lifters jump back a lot while others can keep the bar path more vertical, it depends on the individual. Good luck!!