This MIGHT be the problem... your hips do come down as you bring your shins in place, otherwise they'd be at full-standing height already and your legs couldn't do any lifting. SHOULDERS past the bar, SHOULDER BLADES directly over the bar (I think if you want to nit-pick, it is actually the upper edge of the shoulder blades, but worry about that when you're pulling 250+)
Yes, that explains the bruising. The confusion in that thread was between "unlock the knees" and "bend the knees" - you can unlock them just barely... just don't bend them.
I recommend you practice a squat failure in your rack with an empty bar - this will keep you safe and, more importantly, confident.