Hi nyiregyhazi
Regarding distributing balance, as you move significantly off center toward the outer reaches of the keyboard compass, the left leg in particular (if not engaged with the soft pedal) needs to help balance and brace the body. So for things happening in the bass, it will by necessity have to move the left leg to the left to provide more support support and security for the trunk. Sometimes too it will need to move behind the right leg to add support for playing in the high treble for instance. Again, never have I asserted that the legs must be locked stationary in "home position" (unless both soft and damper pedals are in use leaving little choice). But once support is no longer needed, the left leg is not going to hold position, but will instead move more to center again. Again I think there is a basic misapprehension at play here. I sense you're thinking of my posture suggestions in freeze frame. Not at all! The freeze frame unfreezes as the body has to make natural motions and adjustments to accommodate the demands of the score. But the return is always to approximately to neutral position.
Regarding parallel thighs and/or arms: First, I cannot deny that people are different. If my body proportions dictated that a choice be made, I would always choose arms parallel to the floor, as that is where the risk of carpal tunnel lies. That is, I would adjust accordingly to avoid the greater risk.
Given the pieces you named as examples, I might open the arms angle to 130 degrees or so, but certainly not 180. I tend to sit with my knees about two inches underneath the key bed, which is quite common. Yes, in certain passage work I might have to lean back a bit, but usually not much. A time when I do lean "way back" as you say, is to play ppp or pppp to get a floating sensation in the arms which assists in attaining that dynamic. Similarly if playing fff or ffff I draw closer to the keyboard. Otherwise I sit with a fairly straight back on the bench.
I have to tell you, just from watching videos here, I've seen amazing up-flexed wrists, caused by up-sloping (toward the shoulders) forearms in turn caused by sitting way too high with no apparent justification for it. Those pianists in my opinion are injuries just waiting to happen. Yes, you might argue that Rubinstein sat high and Horowitz low and neither incurred injuries that we know of. To that I would say I'm glad they were fortunate.
There is plenty of flexibility needed to play the piano. I grant you that, and have given several specific examples above to illustrate it. But that that's not to suggest that we ought not have a sitting posture that serves as our ideal for efficiency, comfort and safety. In a piece, we might have to deviate from our ideal a 100 times; but once a pianistic demand has been answered, we should then return to that ideal if the composer allows us to do so.
My sense, as I read and respond to your inquiries, is that we're not as far apart on this issue is you might believe.
David