Thanks for sharing those squat videos. I enjoyed them.
Wow! that first one for 529 was simply awesome! Your a beast - a bearded one at that.
One thing I see from that video is that you are throwing your knees outward on the way down - maybe before you begin to sit which might be causing you to pull your hip forward and tighten your hamstring prematurely. This might even account for the slight tippage in the sticky on the way up.
For me this is/was kinda of fear factor pyscological thing that gets me when I do limits too. When I used to do this often my knees got a bit painful and popped all the time. My lower back would get extra worked also from the pre-activation of the hamstring. As a note: you don't do this in your 415 squat probably cause you feel more than confident sitting into the weight and because 415 is "light" for you.
Here is another way of saying what I was attempting to say:
I had this friend who taught me to use a leg extension machine. He told me it was the best machine ever invented. I soon learned that he blew his knee out squatting for a very heavy limit. He showed me how to use the leg extension machine to target specifically the rectus femoris quad - this is the one that pulls the knee joint from the center. Now, what is most interesting is that I had chronic slight knee pains/hamstring flexibility issues from squatting during my previous years of training. Doing this machine and modifying my squat slightly made these popping pains disappear within weeks. Seriously blew my mind and humbled me before the Iron Goddess.
I adopted this same feel into a particular form of squat that I was outlining above. It is one where I feel like I am almost tucking my knees inward when squatting down (but they don't buckle inward in the slightest - just feels like it). I notice that I cannot go as deep but my knees get this real strong feel to them afterward. I do this style mostly during my warm-up sets. I actually found that close stance squats with feet parallel (3 to 5 inches apart) that are above parallel in height mimic the leg extension machine quite well. I do these cause it works the muscle and tendons right around the knee very well - they begin to burn on high reps and with heavier ones get a real snappy feel to them. This squat I do mostly for knee power.
As a note: the only time I get knee pains now is when I turn my toes outward; otherwise my back and knees never hurt. But when I turn my toes outward I can squat below parallel with more weight. I did this with my last squat session of 2x15x315 (I had something I needed to prove to myself) and I felt it the next couple days until I did my special close stance squats - bingo, pain gone. I've now learned that trade offs are simply part of the game.
Anyway, I hope some of that helps Steve and I really do hope that you may have many years of painless squatting ahead of you.
By the way, I wish I had your bar. Looks like a real nice one...