well one clue i find is in ep1 when quigon reports on his first confrontation with darth maul. he concluded that maul was a sith, because he was "trained in the jedi arts".
if he had simply said "strong with the dark side" or "powerful with the force", or something ambiguous along those lines, it would be different. however, at least in terms of use of the force in combat, it seems to be implied that the jedi arts came first and the sith then formed (or perhaps splintered) to corrupted their discipline. of course this what i inferred and unless GL quoted to support/refute that observation, its speculation (well grounded imho, but none the less speculation)
but yes the question is still begged "was force knowledge an inevitability?". its clearly suggested that many people experience force-powers as children (quigon's conversation with shmi, already knowing anakins capabilities), thus its clear that people would have used the force regardless. i feel however that they would have done so unwittingly as young anakin did. so imho, unknowing force-use would have existed, but not knowledge of the force.
i dont think the sith would have been able to reach the mastery of the force without borrowing the discipline and labor of thousands of years of jedi discipline and evolution of that discipline. while the sith proved to be no slouches, they have nothing new to bring to the discipline. (some can argue that they invented force lightning, but with no evidence other than the fact that we never see a jedi use it)
also, thanks for the thoughtful input