jd,
I tend to shy away from the typical definition of foot plant and to use the moment that the foot is first fully in contact with the ground. The moment where the foot first touches is pretty meaningless as some pitchers come down toe first, some come down flat and some come down heel first. Firmly planted is a continuum that is hard to determine so what I like to use is the first point at which the foot is fully down. A large percentage of MLB pitchers seem to be in a similar position at that point in their motions.
BTW, I was looking at some stuff and I think the reason that some pitchers lift with their elbows and do the reverse W is so that they can delay their arms such that their forearms are closer to parallel at foot plant thereby resulting in more velocity. I don't think the reverse W is inherently bad. If a pitcher does the reverse W and it gets him to between parallel and straight up instead of straight up then there's not much more risk. If it gets him to parallel or not even there yet at full foot contact then that's where the added risk comes in. JMO.