Did Gandalf make a mistake at Isengard ?

[William Cloud Hicklin]

A Nazgul could have snatched them easily enough, as they sat alone by the gate, while the Ents were working at the north side of Isengard.

Flight time from Barad-dur to Isengard was six or seven hours. It wouldn't have been, as they say, actionable intel.

And that assumes that Grima even was concerned with anything other than the political disaster at Edoras and Theoden's riding to war, as well as the destruction of Isengard. Shock upon shock: Gandalf not dead, Isildur's Heir revealed- what matter small rag-tag and bob-tail? Heck, was Wormtongue even in the loop regarding Halflings and their significance? He was merely Saruman's mole in Meduseld with a Rohan portfolio; Ring-related matters were not within his ambit of need-to-know.

And why would Saruman have done anything of the sort? "Sauron, sir! Halflings right here! Maybe one has the Ring!" Saruman was not about to tell Sauron anything Ring-related if he could possibly help it. He was playing a double game right up to then end.