Excellent points. I am not sure if I will purchase the book at this point, but I tend to think it's a big mistake when people claim that there is some sort of a 'secret' that will work for everybody.
Especially for a field such as ours. How often have you seen people saying, 'well, x's bow arm looks wrong, but he plays right' or 'y's left hand looks quite unorthodox, but he still manages', etc? To me, I think that kind of evaluation, while perhaps not off the mark, comes off as a bit impulsive bordering on hasty generalization. Why? Because every BODY is different. Different forearm lengths, arm length, finger length, etc.
Of course, when something is awkard, it WILL look and sound, inevitably, awakard.
I agree that stretching is actually quite important. We are very much similar to athletes, after all. I have heard somewhere that violinists work as much as, if not more than, basketball players. I am skeptical of this claim, but I'd be curious to find out.