As for the chaturanga question: I think that it gets down to a matter of intention with this pose. In Iyengar yoga, it is a stand-alone asana. In Ashtanga, it is a transition, a part of a flowing vinyasa. The in-between, the YWorks variation, performs that sun salute/vinyasa in a very slow, steady fashion. Though you are jumping back, it is a jump-back with a pause; you sort of stick the landing. So, in order to protect your shoulders, etc, you keep a 90 degree angle at the elbow. In Ashtanga, there is a lot more juice and rebound to the action; you spring through the vinyasa with bandhas engaged (which is protective, if you do it consistently and assertively). I could be fooling myself, but it honestly feels perfectly safe.
Also, personally, the tendency to dip my own shoulders forward come from the action of picking up from a seated position to jump-back. For those of us with less core/shoulder/arm and upper-back strength, you need to use simple physics to propel you legs and tush to the back of the mat. That means that while you coil up through the mid-section, you also have to nose-dive a bit to get the height to clear your legs. If you try to keep your chest as forward and level as you do at YWorks, you won't get very far. At least, I wouldn't.
All of this YWorks chat has actually brought up an interesting (and sort of catty) series of thoughts for me. Did you see this article? I have complained to you about this before. Here it is again though. I'm glad for the teacher training that I had at YWorks. I appreciate the teachers who put up with my crunchy self-indulgence, and made my practice more honest. But I have to wonder if this corporate structure hasn't somewhat diluted the yoga. Since leaving the fold, I've been on a quest for inspiration and definition. I hit a wall with the power-yoga, fitness-oriented, "protect the risk factors" brand of practice. Am I being a snotty ingrate? Perhaps. But it's nice to feel like a true believer again. I'm loathe to claim religious affiliation, but I'm a born-again Yogini since striking out on my own.