It’s not an easy one. Checking the Dhammawheel discussion from many years ago, it’s worth noting that the full commentarial explanation (from DN 1) is as follows:
Tattha kāya nutthāti katamāya nu kathāya sannisinnā bhavathāti attho. Kāya netthātipi pāḷi, tassā katamāya nu etthāti attho kāya notthātipi pāḷi. Tassāpi purimoyeva attho.
Herein, kāya nuttha means “for what discussion might you all be seated”. The Pali also has the readings Kāya nettha, which means “for what here”, and kāya nottha, which has the same meaning as the former reading.
Here the commentarial gloss bhavatha definitely indicates that it is reading attha as second plural, either indicative or optative. Ven @Dhammanando in his comment there is confident it is indicative, but I’m not sure why, it seems to me the optative mood would be more fitting.
Regardless, since as you point out the subject can be singular, it seems we have to rule out this reading, as the verb must be plural.
This leaves us with the reading kāya nu ettha. In addition, this seems the most plausible to me in terms of overall sentence structure. The full idiom is divided into two echoing parts:
“kāya nuttha, bhikkhave, etarahi kathāya sannisinnā sannipatitā,
kā ca pana vo antarākathā vippakatā”ti?
The second part clearly lacks an active verb, making do with a past participle. Reading ettha, the two halves are symmetrical.
Moreover, ettha chimes with etarahi in the kind of reduplicated phrasing that we find all the time in Pali: “here and now”. The whole phrase, in fact, now consists of chiming pairs: kāya/kathāya, ettha/etarahi, sannisinnā/sannipatitā. This doesn’t prove anything, but it is consistent with idiomatic phrasing generally that it is euphonious.
Interestingly, Ven Bodhi translates it in different ways. From the 1970s in DN 1 he had “What kind of discussion were you holding just now, bhikkhus?” and most recently in AN 4.185 “What discussion were you engaged in just now”. In both of these he renders etarahi as “just now”, and the idiom as a whole is not very literally rendered, making it unclear how he reads nuttha. When editing Ven Nyanamoli’s translation, however, in MN 26 he has “for what discussion are you sitting together here now?”, which is more literal, and clearly identifies the “here”.