Saruman was never going to willingly help Gandalf. And what was Gandalf going to do... torture him? Sure, Gandalf got a bit cross when interrogating Smeagol, but an intentional waterboarding was never a tactic Gandalf would have employed.

To say that Legolas was "too late" makes no sense. It's demeaning to him as the epitome of elfen archers.

I feel like there may have been an attempt to preserve the fact of Wormtongue's death by arrow by utilizing the only real archer present in the scene.