“Is that the halacha?
No, and I would agree with your implication that this disproves that theory. “
Im not sure I would dismiss it so quickly.
Tefilin isnt the only machlokes we have.
For example regarding the sound of the teruah the question whether it is a shevarim, (what we call) teruah or both is well known. As to how the question came about the Rosh brings from one of the Geonim (Rav Hai?) as one possibility that there were sounds added to enahnce the mitzva and with time we forgot which was the real sound and which enahmcement.
Another possibility the Rosh gives is there was no standarized minhag and (Abaye?) instituted a standard.
Please correct me if I’m wrong, thats the way I remeber it ill look it up tonight.
This sounds similar to the tehory mentioned by old man.
Yet, if somebody only knew how to blow a shevarim sound (and tekiah) should he do it? (putting aside issues of zecher bealma) I would think it is pointless. Now that it was instituted that teruah is either teruah/shevarim/both. Just doing shevarim is pointless even though it once was ok.
Similarly it is possible that there was no orgiginal standard order of parshiyos but now that there is, just doing any order without a shita backing it is pointless.
(of course the anaolgy isnt perfect, and im not sure just doing shevarim is pointless, does anybody discuss this issue?