Rebdoniel,
Farber quite clearly claims that Devarim is written by a different author than Shemos and Bamidbar. Not in the sense that Devarim is, for the most part, Moshe’s speech which Hashem made part of the Torah as compared to the rest of the Torah which is, for the most part, Hashem’s narration. Rather Farber clearly states that he believes there were “multiple authors with multiple traditions.” He uses different traditions to explain away the contradictions he perceives.
That is worse than even what HaLivni has written and HaLivni is a Conservative scholar and an apikorus according to the Rambam.
I don’t have any particular knowledge to refute your claims about the other Rabbis you mentioned but I am very, very skeptical that they would have agreed with Farber’s statement. It is no small shakes to accuse these men of a position that is clearly apikorsus without citations to sources.