Mostly agree with your points. I would say if you had to reduce frame size to one number, that best number would be top tube length...but it really isn't that simple either. The tube angles (not just lengths) play a role. Head tube length is important as well, but for a given frame size (say a 57 c-t) head tube height is going to be directly influenced by BB height, so you have to consider BB height...and so forth.

The bottom line is...even if everyone were built to the same proportions, correctly sizing a frame is a complex problem...and most people here are clueless about it. You can't reduce it to a single number. However, good bike fit isn't really a point...there is a window of sizes within which someone can ride comfortably and efficiently. In the beginning, the goal really is just to get within that window. Once you are there, lots and lots of riding and tweaking your fit (and riding other bikes!) will slowly help you hone in on exactly what fits you best.

A professional fitting is a good way to start, but its no substitute for miles and miles in the saddle.

My reference to fit calculators was really only as another point of reference. Good fit calculators will spit out more than a single number, which can then be compared to the Rush's geometry.