Before the 1962 reforms, there were no rubrics for the laity at all. From a liturgical law point of view, turning cartwheels was no worse (and no better) than kneeling. Custom and common sense largely held sway, but things did vary from region to region.
There is a great story about Belloc going to Mass in France and kneeling at a time where it was customary to kneel in England, but to stand in France. An usher thought that Belloc was a confused Anglican, and came up to him and said, “excuse me sir, but we stand here,” to which Belloc replied “Go to hell,” to which the usher replied “I’m sorry, I didn’t know you were Catholic.”
I do agree that if the Council has issued a completely revised Missal itself, things would have been less ambiguous. Given that uniform instructions for the laity had first come into force only in the previous year, the most natural interpretation would be that everybody ought to do what we just got through telling everybody to do. Yes, it is possible that different directions might be forthcoming in the future, but I don’t recall anything in the documents to the effect that “the just given rubrics for the laity on posture aren’t working, please change them.” If you see something to that effect, please give me the paragraph number.
Have you read "Iota Unum" or The Ecumenical Vatican Council II: A Much Needed Discussion"?