«Let's go back to the common starting point, Ma'amad Har Sinai. Let's take a hard line stance, that as Moshe Rabbeinu descended the mountain all of Written Torah and all of Oral Torah was MiSinai, meaning Divine, Holy. What might have occurred that would make the common denominator across the Ortho spectrum to diverge over time?
Stay Tuned for future posts addressing this point.»
There are 2 events and several processes that could cause a divergence of The Pristine MiSinai
The second the Torah was "lo bshamayim hee" when leaders [IE before there were rabbis] started interpreting Torah. Here are 2 POV's WRT to that process
A. Since it is authorized by the Torah itself - A Bona Fide Sanhedrin [BD Haggadol] has the same authority as Sinai
B. Let's face it a Sanhedrin may be commissioned by the Torah itself in Parshat Mishpatim, nevertheless a human element has entered - thereby diluting the Original Sinai quotient.
Thus According to Model A: Anshei Knesset Hag'dolah [AKhG] - which included nevi-im - was indeed miSinai. This explains the ability to make a G'zeirah Shavah [Prazi Prazi see TB Megillah]] between Megillat Esther and Humash - even though GS's normally should stem miSinai only. IOW, AKhG equates to miSinai - because they canonized Megillat Esther; AND therefore COULD create a bona fide Masorah for any GS.
We can see however, that this term MiSinai might imply 2 levels of understanding already in the AKhG era.
1 the Fundamentalist literal read suggest by Jacob Agus - as Divine
2 the looser read suggested by R Shimshon of Sens suggesting both antiquity and being axiomatic, but not necessarily literally Divine in nature.
[NB: This looser read is fundamental to R Zecharyah Frankel's take on TSBP in general]
These interpretations may even have d'Oraitto authority in a Halachic sense, but hashqafically we may see them as diverging from Pristine Sinai the Divine.
Next phase, Post-Hurban and its aftermath. For a sneak preview see Rambam MT Mamrim 1:1