If you don't have a monolift you'd be better off setting the bar on the safety pins and just having your spotters pull the pins out after you stand up. Getting out that wide with a walk out is dangerous and stopping you from ever getting a good set up where everything is locked.
The 2 points I'd look to are your break and your chest. You are breaking straight down and then trying to slide your hips back - that'll never happen in gear. As you add more gear it's just more pressure pushing your hips forward crumpling you up.
Start locked, chest up and hips in. Start your descent by pushing your hips way back. Once you've pushed back enough that your knees start to bend, stop pushing back and start pushing your knees out harder.
Picture pulling back on a rubber band as you squat. Those briefs are loading up as you sit back in to them. Once your hips come forward you've spent all that energy they had stored up. Training box squats will definitely help. Wall squats might help technique with this as well.
There's nothing wrong with the bar being that high on your back, but if you're going to squat like that you'll need to keep your chest up and stay more upright or you're just going to end up way forward trying to good morning the weight up.
For high bar squat form, look to guys like Jose Garcia -
http://youtu.be/mAc2Mj7rec4
and for a mid-high bar look to guys like Justin Graalfs...
http://youtu.be/FnvRl1kUEo4
Or, Vogelpohl used to be low bar, but is now high bar squatting and looks picture perfect...
http://youtu.be/a8_ksbYVRwo