Like you said, the closest the story gets to describing Columbia is second hand. We have no idea how accurate it is.
Someone call Gregory Maguire. As I read the last quarter of "Wicked", and watched Elphaba's descent from what she was into what we know from the original Oz book and film, I really felt as though Maguire's portrayal was more accurate.
I wonder if the author has any plans for flash or other short fiction about Columbia, maybe an origin story for Brad and Jody.
I think a truly great author can write from the other side and make it seem completely believable and biased in the opposite direction (for example, if I, as a conservative, were to write a liberal character, I would have to consult my liberal friends to make sure I characterized her properly). I -- and apparently most of the rest of the folks on here -- want to learn more about Columbia, probably from the POV of a Columbian. Mike, after all, was a Confederate, and he told the story from the Confederate side. One wonders if the Columbian would have similar opinions and attitudes.
One thing I will say is that, after reading all the debate in this forum (and trying to remember all the points I made back on page one), Ms McDonald's story seems very believable to me in that I'm sure many of the more-extreme individuals out there (ie: the ones who are incapable of rationally discussing the topic, and I am not talking about ANYONE on the forums, but instead the more foaming-at-the-mouth types we all know and vilify regardless of our personal opinions) would take one look at this story and say "wow, this seems like a good idea, and something we can aspire to." Not the activities Mike talked about in the story, but the fact that the US could be split into two (or more) viable nations.
Stories like these remind me of a great quote I read back in the 90s:
"The metaphor of [America as] the melting pot is unfortunate and misleading. A more accurate analogy would be a salad bowl, for, though the salad is an entity, the lettuce can still be distinguished from the chicory, the tomatoes from the cabbage." -Carl Degler, "Out of Our Past"