Why are we assuming that ST:INS took place during the war? Or at any other specific time, for that matter?
The movie has no stardates anywhere. It doesn't feature any references to any events from any other movies or TV shows. While the Son'a get mentioned in DS9, there is no pressing reason to think that either the INS or the DS9 mention would precede the other in the in-universe chronology.
It thus seems natural to assume that the movie takes place some time after the war. That way, our TNG heroes can freely perform long missions of zero military worth without being shot for deserting. It also becomes more plausible that they completely fail to mention the war. And the plot makes more sense: the UFP can be in cahoots with the Son'a, while during the war it would be impossible to fraternize with somebody who aids and abets the Dominion; and the UFP diplomats can be negotiating with the Dominion, which they certainly wouldn't while the fighting was still going on (since the Dominion had demonstrated such complete lack of trustworthiness in "Statistical Probabilities").
IMHO it was a splendid choice not to insert any cross-references between the movie and the various TV shows. Not only did it give dramatic freedom to the people writing the TV shows, it also brought credibility to the INS plotline that simply wouldn't have been there if the writers had insisted that it take place during the war.
Timo Saloniemi