Oggers,
The Columbia situation is the same point point again there's wood to focus on before worrying about the trees. You don't need to do irrelevant sums, you can just say that different sums need to be done. You don't ask for sums at a PhD exam, you ask for insight, intuition and an understanding of the issues.
Here's one reason why - and one is sufficient to drop the point and think more about it.
Vertical changes in gravity by 1% are a minor detail, since the break up was not at a point - it took tens of seconds, and spread over tens of kilometers. Thus your uncertainty in initial position makes a potential 1% shift in a 700km track neither here nor there.
Remember that the drag for the whole shuttle brings it down in Florida about 4000km after substantial contact with the atmosphere, and the actually distance from break up to debris hitting the ground was only 100-200km, so saying anything numerical "without drag" is like focussing on a flea while missing a bear it is sitting on.