I understand the SN 12.2 is an example of that pro-abhidhamma analytics. Most times the Buddha taught namarupa although in this Sutta we find how in the use of the term these meanings should be understood by the disciples:
"And what is nama-rupa? Feeling, perception, intention, contact & attention: this is called nama. The four great elements, and the rupa dependent on the four great elements: this is called rupa. This nama and this rupa are called nama-rupa"
if we understand rupa only like nama, and it would be enough for the progress then where is the necessity of such distinction. Despite we should know rupa representations the distinction should be understood because are not the same. The distinction should arise to be realized by means wisdom.
I fear that many of these issues sounds very different depending Suttas or Abhdihamma although most times is more the difficulty with the inheritance of the later explanations. However, the expansion of explanations is something unavoidable in the development of human knowledge after some foundaments are given. Probably we could find Suttas in where some disciples explain more details or more extensions than the Buddha himself teaching the same issue.
The momentariness is a similar issue. At least I don't see that opposition that many people sees. The momentariness is a resource for the reason, although it doesn't mean it is non-real or non-existent. Our mind should grasp the objects of knowledge with two references, space and time, and both are linked with the existence of the knower, -self. We never grasp "one moment" in itself but because the knowledge of its objects of reference.
When there is grasping of two objects the conditionality is known, and also the moment becomes real. It is a fact. The Time don't inhabits inside the clocks but in the mind. Nobody can deny the Time and that the moments are real; this is our experience of Reality. From there, also is born the possibility to know more details of the process and factors involved in the arising and vanishing of objects which are building the instant. in that way we become closer to the building of atta. These can be known, analyzed and so on; giving rise to new words and relations and to further expansions of the same issue.
The Atthasalini starts:
"By Time the Sage described the Mind
And by the Mind he described the Time;
In order, that by such a definition,
The dhammas there in classes may be show"
at least to me, these verses are interesting even a bit funny. Because it seems like if "the Sage" would apologize when he should dive the Truth into the conditional world. To do that there is the causal necessity of the Time, and logically he explain the Time being a causal means in the conditional Reality, to start the explanation of our reality of grasped objects (dhammas). Time is born from the grasping of objects, in our conditional Reality. Without Time there is no "before" and no "after", there are no moments, no dhammas and no concepts would be available to be explained. Without Time there is no grasping neither conditional reality. In nibbana there is not Time.
Discussions around the momentariness arose in an Historical moment in where the sophistication of the intellectual explanation was necessary. The world and the environment was different. Although, of course, before that time everybody was aware about the existence of the "moment". This a logical thing. Thinking the contrary idea would be an absurdity. Although, Why not the same development existed?
Perhaps we could make an analogy with the arising of the zero number in the West. The zero number was non-existent through many centuries in Europe. And logically it doesn't mean the concept of the absence of something was unknown before the zero number adoption. Neither it means that the concept of "absence of" was not analyzed. There are analysis from the ancient Greeks (the "keno", previous to the arising of the atomist discussions). However, the zero number arose because the necessity to improve the understanding of the Reality according the mathematical thought.
This happened mostly because texts, commerce and Economy. The Europeans realized that the Roman numeration system was inferior to the Arab numerical system, who inherited that from India. The zero number allowed more powerful calculations by means a deeper understanding of the Reality in mathematical terms. However, before that time, everybody knew the idea of the absence of something.
Just it happened that before the arising of the zero number in Europe, the people had another use of the matemathical thought, and it was no necessary to define the absence in conceptual ways like another number to deal with the Reality. Although it worked very well. Everybody knows the amazing technique of Romans; still their buildings are more solid and beauty than most of present ones. And when we contemplate a Roman arc, also the space was obvioulsy analyzed and understood.
So at least I believe this issue is quite similar. Like many scholars writes, the Buddha Sangha was composed by groups of disciples with different tools and approaches. And of course also the proto-abhidhamma disciples existed. Because the same analytic approach we see inside the Abhidhamma is not an historical invention but the development of a distinctive position for the kwnoledge in the human being. Existing from always. It is another way to focus the Reality and the progress: by dissecting the grasping and arising and vanishing. If we deny this approach existed, also we can fall in the absurdity to deny that this characteristic of the human knowledge was absent in Buddha times.
I believe - just a personal view - that in the later historical discussions the issue was to give a definitive form the original proto-abhidhamma and to establish a definitive Abhidhamma. However, also were times of a decreasing of arhants. And lesser ariyas cannot review with enough stability those instants of leaving the conditional reality into nibbana. And then probably we find those historical discussions between different schools like an effort to have the better exact schema for the understanding.
Lamotte mentions in his "History..." how the discussions between those later schools were philosophically intense, although peaceful for the rest. They were sharing common spaces even the same monasteries.
In short, I think the momentariness is an explanation to understand the conditional reality and to get detachment. There is people who is more kammically akin to that approach while other have another kammic trend. Same happened in Buddha times, and in that way we read about the different titles according their approach for the different ariyas. That's the kamma and the human variety.