Filling gaps is a perfectly fine exercise, and can even lead to consensus when the gap is small and you take years of discussion to fill it. The question of Sauron's anti-air detection and defense capabilities within Mordor, and of the Council of Elrond's confidence in assessing these capabilities, is much larger, vaguer and conducive to controversies. Not to mention that the character most likely to propose using the eagles (Gandalf) was also the most aware of any possible impediments (e.g. Sauron being able to conjure storms and blinding lights at a moment's notice etc.).
