Hi Gabriel
It’s a little late, but I hope this can still be discussed.
I agree that virāgā is ablative. It seems to me that all of the translations have consistently rendered pītiyā here as a genitive, giving therefore “with the fading away of zest”.
Here, I’m not so certain that the ca is acting like a copulative “and”. It appears to be functioning like “moreover”. Oddly enough, the ca is not found in a Sanskrit parallel -
Sa prīter-virāgād-upekṣako viharati | smṛtaḥ saṁprajānan sukhaṁ ca kāyena pratisaṁvedayati |
Something similar also here - https://suttacentral.net/skt/lal11 and other Sanskrit texts that I found on SC.
There was an extensive discussion of this instrumental kāyena here - Touching enlightenment with the body
Expanding on my previous post about the polysemy of ātman, what seems to have happened is that while ātman’s meaning as “body” was gradually supplanted by kāya, in turn kāya retained much of its older connection with ātman, especially the reflexive use of ātman as “myself”, “himself” etc. This seems to show up in sakkāya.
Kāya, in fact, veers very closely to the older Vedic meaning of “Self” in its appearance as manomaya kāya, where the kāya here is typically interpreted to refer to a physical body. But, a quick look at how a synonym is used in DN 9 should dispel that notion. There, instead of manomaya kāya, we have manomaya attapaṭilābha (mind-made acquisition of self). You will see that the definition of manomaya attapaṭilābha in DN 9 is the same as the definition of manomaya kāya in other suttas. So, it would seem that kāya does hark back to its Vedic roots as ātman.
Of course, the Buddha ends by suggesting that this “acquisition of self” is just a worldly convention, which He uses without grasping to it.