Irlandes,
The formula is a simple translation of the speed change that the rotor must make on a column of air in order to make the hover thrust. It is only an approximation (a good one) but it is restricted to the case of hover. The factors that you describe are of course necessary, but they tend to describe the details that operate the wind machine. In the end, small angle of attack on a high speed rotor, or large angle of attack on a slow one all must move the air, and it is the net momentum change on the air that determines the thrust of the rotor.
In a climb or descent, the downwash speed is the same, as long as the ROD or ROC is steady state. Recall that lift equals weight as long as the flight is unaccelerated. It takes a bit more thrust (downwash) to initiate the climb, and a bit less to start the descent, of course.
I will make this change to the web site to clarify the limits of that formula.
Thanks!
Nick