Danuser said in today's Q&A; the Anduin was needed to get an audience with the Archon (and obviously, getting the key).
Did Sylvanas really believe she was able to convince Anduin to attack the Archon? Or the plan was to convince Anduin and then to convince the Firsbone to bring suicide? Obviously, this is not the case. How Anduin left the Kyrian territory without any harm?
I am also amazed the Kyrian are so adamant about receiving a soul like Uther, but if it is a living good human king that was trapped...they low their defenses a lot...
Sylvanas seems quite naive in the story: not noticing the Jailer created Arthas, no realizing that she is another pawn, and now with Anduin.
I hope Blizzard brings a world-changing reason for "convincing: arguments of the Jailer.
TL;DR: Sylvanas wanted Anduin to collaborate voluntarily. There was NO way to convince Anduin to attack the Archon. The only way was the Jailer to control him.