On the grounds of contemporaneousness alone, I've always agreed w/ this point. Even Rishi Vyasa's composition - Adhyatma Ramayan - came a yuga later, and most of the vernacular translations by Kamban, Krittivas, Tulsidas, et al came during the middle ages, when the original stories couldn't possibly have been in mind.
I once read somewhere that Rishi Vashistha too had once written a version of Ramayan. If that's ever established, and if what he wrote could be authenticated as best as possible, that would be the only legitimate account of events other than Valmiki.
As to what parts of Valmiki itself are adulterated, I don't know about that one, so have no comments on it.