The context isn't fully spelled out, but Powell is describing the inspiration for "The Interrogative Mood: A Novel?" Those lines are the beginning of his novel and the novel itself is entirely in the form of questions.
So he's just mocking the melodramatic message of his colleague? As someone who's not that well read, I find his response nonsensical, so I imagine I'm missing a reference or two.
I may be reading too far into it, but I think he _is_ mocking his colleague by responding to questions with similar questions.
> Are your emotions pure?
Are you thinking through this with clear thought and emotion?
> Are your nerves adjustable?
Do you feel so strongly about this that it has affected you this whole time, since spring?
> How do you stand in relation to the potato?
Can you not support yourself? Are you as wellrounded or less so than a potato?
> Should it still be Constantinople?
Can the outcome not change even when history repeats itself?
The last one to me seems like the indicator that these questions are not nonsensical. As it is is direct reference to the colleague's question about history repeating itself and refers to a location which changed back and forth numerous times.
