Hi! 
It is commonly understood that the word upādānakhandha has multiple meanings, dependent on context. You are understanding it only as khandhas which are currently clung to or grasped, which is true in some contexts. But in other contexts, such as the truth on suffering, it means khandhas which are the result of clinging or grasping (in a past life). (I.e., the compound contains an ablative case, causal relation, “the aggregates due to grasping”.) Venerable Bodhi says about this, in his introduction to SN22: “Clinging to the five aggregates in this existence brings forth a new birth and thus the reappearance of the five aggregates in the next existence.” Venerable Sujato likewise notes in his intro to the Linked Discourses: “They are the product of grasping in the sense that attachments in past lives have given rise to the aggregates in this life.” This sense of upādānakhandhas is known to the commentaries as * upādinnakhandhas*, ‘khandhas which were clung to/grasped’ (or, as I prefer, ‘taken up’).
There are texts which indicate the enlightened ones still have the upādānakhandhas, where this sense is by far the easiest, if not only, way to interpret the term. For example, SN22.122–123 say even enlightened ones should still see the upādānakhandhas as suffering. MN112 says the enlightened mind is liberated by not grasping/clinging to the upādānakhandhas. SN22.85 similarly says: "These five aggregates of clinging, to which he does not become engaged and to which he does not cling.” Here it surely can’t mean ‘khandhas which are currently clung to’.
That in the first noble truth it also means khandhas which are the result of grasping, is probably most clearly illustrated by SN56.14, which says: “And what, bhikkhus, is the noble truth of suffering? It should be said: the six internal sense bases.” There is no mention of clinging here. It simply says the six senses themselves are suffering. Compare this text with the preceding one, SN56.13, which instead talks of the five upādānakhandhas. Dhp202 also says, without any mention of clinging: “There is no suffering like the aspects of existence.” Thag19.1 likewise: “Reflect rationally on the aggregates as suffering”, without referring upādāna. Similar at Thig16.1. See also SN22.22 which once says the upādānakhandhas are a burden (i.e. suffering), and once that the “mere” khandhas are a burden. Here the two words are used synonymously.
It doesn’t say that, though. If anything, the upadana-khandhas in this sutta are a subset of the khandhas as a whole, because the latter aren’t described as “without grasping”. Bhikkhu Bodhi explained this extensively in an article on the sutta, the name of which I can’t remember.
There are so many contexts that imply things to be suffering without implying grasping or other defilements, I almost wouldn’t know where to start. But a good one is SN36.11, which says, "Suffering includes whatever is felt.” Whatever is felt means all feelings, whether grasped or not. To read this in any other way seems overly inferential. The Buddha even says directly afterwards, “when I said this I was referring to the impermanence of conditions”. That is, all feelings should be seen as suffering because of their inherent impermanence, not because of grasping.
There are also suttas like SN22.30, which says: “The cessation, settling, and ending of form is the cessation of suffering, the settling of diseases, and the ending of old age and death.” You would have to read this as “the ending form that’s grasped to, is the cessation of suffering”, which seems a very awkward way to read it. Also, since after enlightenment there still is disease, old age and death, how is the end grasping the end of these things? It would again have to be taken very non-literally, if anything.
I’d also like to understand what you think the Teacher meant by citing the craving for non-existence
Craving for nonexistence of the aggregates also needs to be abandoned, even if you see them as suffering, because all craving comes from a sense of self. Enlightened ones are equanimous about suffering.
I’ll leave the details on the “BTW” on form, because it seems tangential, and, indeed, no sutta explains it directly, so it would take me a while to do so. Let’s for sake of argument agree that rūpa only means physical body and not also awareness of certain things, including the body. You say it implies, “the body, when grasped, is suffering”. I would then say it implies something like, “the body, when aware of, is suffering” or “the body always results in suffering”. This seems to be much less presumptive to me. SN22.79 explains by means of a pun: “And why do you call it form (rūpa)? It’s hurt (ruppati); that’s why it’s called ‘form’. Hurt by what? Hurt by cold, heat, hunger, and thirst, and hurt by the touch of flies, mosquitoes, wind, sun, and reptiles.” It doesn’t say it’s hurt by grasping. (Other translators translate ruppati non-literally as ‘deformed’ in an attempt to show the pun in English, but that misses a lot of the meaning.) Likewise, MN74 says that the body should be seen as suffering, as a disease, as a tumour, as a dart, as a calamity, as an affliction, for it is of the nature to decay, not because we attach to it.
And to get back to the topic, when SN5.10 says: “This is just a bunch of created things (saṅkhāras). There exists no being as such. […] It is only suffering that comes to be, suffering that exists and vanishes.” If suffering equals grasping, is it just grasping that comes to be and vanishes?? Are saṅkharas just grasping?
