Krak: I'm pretty sure, that up to this point in this thread, that nobody has yet attempted to answer the question you actually asked! It doesn't seem all that complicated! I can sense your frustration.
I agree with you. A fastball in the 60's, it probably doesn't matter how well placed, or even how much movement on it, or even how good his other pitch is. It is slow enough so that hitters will be able to adjust.
I do think that a kid can do well in the low 70's though. My son had a teammate who threw VERY successfully in his soph through senior years ranging in the low to mid 70's. He was league pitcher of the year in his soph and jr years. He had two different curves and could place his fastball.
So my answer is essentially same as yours. Anything under 70 and I think it doesn't matter how good his control or how good his second pitch (unless that second pitch is a 75 MPH curve!)