Speaking of which, in reading your post Dan I began to wonder, how does one know that there is sufficient built-in structure to begin training with force?
Uhm...by failing when you get a little force.
It's a life long study. It's not like you ever finish. And its not just this pushing stuff or what to do with the upper cross, or tailbone or with just with frame work. Using it is more complex. And takes years of work. Each step is a lengthy study; of what supports what, where, why and how, what works better overall, what works better for your needs, and so on. as many are finding out though its a whole lot of fun. I have had some teachers call it grad school for Aikido teachers, or a second beginning with a twinkle in their eye and a smile on their face!