As I stated, she had back-up plans lest Plan "A" failed. How could she be a devious leader if she didn't prepare for such? Though wily and able to counter nearly everyone's movement, it was her friends she didn't predict; stemming from an ego that they feared her too much to leave her, and a hidden cling that these two truly support and love her. It was a rare occasion where she get let her shield down in front of the two without them realizing until the last minute. Abrupt in the end? Yes, I agree, but in a long run, it was possibly intentional because it had befallen on Azula so quickly, so fast that she could not react with the appropriate behavior.
Honestly, if it happened during mid-Season Two, then we wouldn't have seen some of her most awesome during the Season Two Finale; it was required to show how one chick (and two partners) could conquer an entire kingdom by her lone self. I'd much prefer if they started a bit earlier in Season Three. Technically "The Beach" held some clues, but that episode felt as though it was throwing out every one of the characters' emotions in one burst for me to accept so easily.