@frankk
I am not sure if the exact interpretation of kāya here makes much practical difference. If you go with experiencing the entire physical body, the breath will be an important part of that. If you instead focus on the entire breath, the body will still be there in the background. I don’t think there is any vast difference between the two, but more like a difference in emphasis and degree.
Differences in interpretation become crucial when we are dealing with the goal of the path, or substantial sub-goals (e.g. jhāna). But sabbakāyapaṭisaṃvedī, the third step of ānāpānasati, is only part of a much bigger process, and it is largely defined by the steps on either side of it. If the overall process is heading in the right direction, then getting the exact meaning of the third step is not so crucial.
But since there are bigger issues around the meaning of kāya, it may still be useful to discuss this in greater detail.
Perhaps, but they are not independent of each other. If you are focused on a movie to the exclusion of all other sensory input, you have a high degree of non-distraction, which is one of the qualities of samādhi. But this would not qualify as samādhi from a sutta point of view. Samādhi, as opposed to mindfulness, requires a high degree of focus. If there are too many things going on, there is no proper focus, no convergence (perhaps Bhante Sujato is on to something!), and no real stillness either.
This, I think, is asking too much of language. Language is messy and convoluted. Words have multiple meanings that evolve over time, and there are multiple ways of saying the same thing, each with slightly varying emphasis. The above argument could be used in almost any context, and for that reason it is really no argument at all.
In fact this is what we see in one of the Sanskrit parallel quoted below by Bhante Sujato:
Sarva-kāya-saṁskāra-pratisaṁvedī āśvasan sarva-kāya-saṁskāra-pratisaṁvedī āśvasāmīti yathā-bhūtaṁ prajānāti | sarva-kāya-saṁskāra-pratisaṁvedī praśvasan sarva-kāya-saṁskāra-pratisaṁvedī praśvasāmīti yathā-bhūtaṁ prajānāti
We don’t know which is more original, if any, but it is all pointing towards the breath. If the Pali version is more original, then the Sanskrit version shows us that the early Buddhists interpreted sabbakāya as the whole breath. If the Sanskrit version is more original, then this is even more clear. Moreover, the whole sutta is about mindfulness of breathing, not mindfulness of the body.
Monks, and others, would have memorised different things, and it is quite possible that some would not have remembered even the basic sixteen steps. The Buddha teaches in brief and he teaches in detail. As you point out, many would have just remembered the brief statements. But this does not mean that the detailed explanations are irrelevant. It just means that if you had any doubts you would go to someone who knew more than you and ask for advice.
The extra information in MN118 is actually very significant and it is attested in other versions of this sutta. It is this extra information that makes it clear that ānāpānasati on its own can take you all the way to awakening, via fulfilling the satipaṭṭhānas and the factors of awakening. As part of this it explains how watching the breath fulfils the contemplation of the body. The answer is that the breath is itself considered a kāya (“conglomeration”), among all sorts of kāyas.