Nick, it seems to me that you may have the situation exactly reversed. Were that the case, propositions would have been contemporaneous with concrete entities that comprise the corresponding states of affairs.
Fregean propositions are general, unlike the singular Russellian variety.
My suggestion was that this motivation was epistemic. You are confusing ontological separation of concepts from the entities represented thereby, with a separate issue of logical independence of the former from the laws that govern the latter. Laws of nature are abstract entities existing on par with all possible contents of belief. Any reasonable belief closure conditions will subsume some regularities associated with its objects. It is quite beside the point whether or not these regularities are necessary or contingent, though Platonically inclined thinkers refuse to distinguish between logical and physical necessity.