Derek,

>> The fact that the commentator contrasts saddhaaya with a string of

ablatives suggests that he thinks that saddhaaya itself is also ablative.<<

I assume you were speaking about this passage:

>>saddhaaya gharaa nikkhanto na raajaabhiniito, na coraabhiniito, na

i.na.t.to, na bhaya.t.to, na jiivikaapakato<<

These are all nominative singulars. I just went with the instrumental
because ablative wouldn't make much sense (because of gharaa as an
ablative).

>> One question that occurs to me: To a native speaker of Pali (if there

ever were such a thing!) would this distinction really exist, since the word
forms are actually the same in Pali?<<

The key here is context (and also probably other things such as stress,
tone, etc.). English has very few inflections, but we get by just fine
using either extra words, or context (such as when you say, "There's their
car."). English has a lot of words that sound the same but it is the
context that gives the meaning. We think, 'because of these surrounding
words it has to mean this,' or, 'because it was stressed in this way this is
what it means.' This usually is a given as a native speaker. That is why
saddhaaya grouped next to a gharaa (abl.) couldn't be very intelligable if
it was also abl. (because they be seen as a closly related to each other -
such as tena samayena).

>>> But then again, if the verse was not originally cast in Pali but in some

related MIA dialect, would the instrumental and ablative forms in that
dialect also be the same?<<<

Having studied the Asoka edicts quite a bit I can say they all featured the
'aa' inflections as 'ya' (or 'ye'). The Northern Indian dialects are
surprising similar in their general structure. (That is why I think that the
Buddha was able to travel all over Northern India and have little trouble
speaking to the people, but that's another topic...)

Just sharing my thoughts, no big wop.
Sean