A few points
1.>The planet was ceded to the Cardassians as party of a peace treaty, some cardassian worlds became Federation worlds.
2.>The Inhabitant's were Federation citizens
3.>With the Ba'ku the orders were based on a false assumption. Once it became known that the Sona and Ba'ku where the same race. the Prime Directive kicked in. i.e Starfleet officers should take all steps to avoid becoming invovled in the internal affairs of other races.
As for whether or not the Cardassians intended to keep their end of the treaty is a moot point. Are you really trying to argue the point that the UFP should perhaps force a war that could cost millions of lifes just because the Cardassians MAY not keep the treaty.