Attempting to date a book based upon what historical background best fits is rather elusive, as numerous historical backgrounds from different time periods would work just fine. Likewise, attempting to date books, proto-books, or sources based upon orthography is also dubious. For instance, take this interesting observation by Adele Berlin (1994:41).
Linguistic criteria present the same ambiguity as historical criteria. According to their orthography, all the books of the Twelve should be dated to the same time--the sixth century--owing to the influence of the process of compiling the Twelve. Yet no one would say that all twelve books were composed at the same time.
The author does go on to make a case that more study is needed in applying Polzin's analysis of Late Biblical Hebrew in prose to the Minor Prophets, but the point one can make is that philological variations can be regional rather than chronological, they can be idiosyncratic to certain groups, they can reflect archaizing, they may be associated with particular genre, they may exist as carry-overs from whatever sources were used and not an indication of when something was written or included in a work.
All in all, the constant shifting and disagreement within scholarship is largely reflective of the fact that the criteria used to judge the date of a book or whether its sources were added earlier or later or are all a part of the original composition are not definitive by any means; and this means that one should remain humble in using this data.