"The Spirit can only 'reveal' the amount of 'truth' that humanity, individually and collectively, is able to comprehend at any given time."

Yeah! Fundamental insight, Leo! 

Rather obviously the core difference between traditionalist and progressive Catholics is the concept of development of doctrine. And your insight goes to the core of the matter, for it explains why doctrine must develop; why what our ancestors understood the Spirit was revealing to them is always going to different from our understand of the same Spirit. It is not the Spirit who changes, but us.

It also reveals, I suggest, something that progressives overlook. It is not enough to simply change our theology--we must explain how the new emerges from the old, for it is always the inspiration of the same Spirit that lies behind the old and the new. If this or that piece of new theology can not be shown at some deeper level to grow organically from the old, the odds are increased that the new is somehow off kilter.