Hi Bryan,

You wrote:
<< The commentary states

mu.n.dakena sama.nakenaa ti mu.n.da.m mu.n.doti, sama.na.m vaa sama.noti
vattu.m va.t.tati, aya.m pana aparipakka~naa.nattaa braahma.nakule
uggahitavohaaravaseneva hii.lento evamaaha. (3.280)

which I tentatively translate as

mu.n.dakena sama.makena: It is proper to call a shaved person a "mu.n.do" or
an ascetic a "sama.no" - because of using these terms, this person [i. e.
Jotipaal] spoke thus, looking down upon [those] whose knowledge is not
developed in respect of a brahmanic family.

I find the commentary a bit confusing and the grammar awkward (unless of
course I am misreading it which is very possible) but it appears that the
commentator is explaining why Jotipaal (who is a brahmin) is calling the
Buddha Kassapa these names (mu.n.da and sama.na).

Is the form aparipakka~naa.nattaa in the ablative?
(Skt.aparipakvaj~naanatvaat?). >>

Yes.

<< Is braahma.nakule in the locative or could this be acccus. plural (object
of hii.lento)? >>

Locative singular.

This is how I would translate: "aya.m pana aparipakka~naa.nattaa
braahma.nakule uggahitavohaaravaseneva hii.lento evamaaha."

But he (Jotipaala), from there being an opposite understanding through the
influsence of terms acquired in a Brahmin family, looking down, spoke thus.

By using the diminutive suffix 'ka' after mu.n.da and sama.na he is showing
disrespect.

Best wishes,

Jim