I'd like to just follow up on my own post, since I didn't see any replies..
and I trust you'll all forgive me if I'm unable to describe what I mean all
too eloquently - my understanding of grammar is more 'street smarts' than
it is book smarts.

However, I do think that there's a distinct possibility, as I suggested
before, that "what is it" is in fact itself the 'overwhelming question',
rather than a response to the mooting of the notion of said question.  If,
as Carrol suggested, Prufrock is indeed talking to himself (and Eliot has
been vague, and contradictory, on this point), then it's an especially
compelling idea (to me, anyhow) - the "do not ask what is it" gains a real
urgency that's lost if it's read more simply as "look, don't ask me what
the question is, but let me show you instead."  It's no secret that Eliot
enjoyed playing word games, and wrapping meaning up in a few layers --
perhaps, then, we can read this as something of a cryptic clue: if it's a
matter of an "argument", then there's the suggestion that the following
lines are spoken by different voices; and the word "insidious" puts one
one's guard for trickery.

To what end?  Well, if "what is it?" is the question, then it's a question
which requires an emotional component - an empathy between the questioner
and the questioned.  And if there's anything that overwhelms Prufrock, it's
that kind of interaction...

Anyroadup, I'm rambling away again when I should be asleep.  Does anyone
have any thoughts on the matter, or am I just getting carried away by a
daft idea?  It wouldn't be the first time.