Nice article, Mark. An interesting idea with inclusion/exclusion, which I'm looking forward to trying out. Isn't there a risk that one ends up at somewhere between "optimum image" and "falls apart" if there is a slightly less that optimum composition in between? Or when you say "falls apart", do you mean "becomes noticably worse"?
It doesn't surprise me that the left-to-right falling image was good. Yes, in many other contexts a left-to-right rising diagonal is preferred as it implies growth, positivity etc, but for a waterfall we expect, well, a fall. So when we (Westerners) read it left-to-right, having a rising diagonal is counterintuitive. Unless we happen upon one of the rare waterrises:)