I have defeated the real messenger behind the portal chamber. The fight was pretty easy compared to other bosses, even the usual villain exposition was cut to a minimum. He even said that he has no answers for me and indeed he didn't provide any. In the end he just laid there, unconscious, no clues found by searching his body either.

Yet suddenly Eilenai knows who is behind all this, what he needed the stolen book for etc. After being clueless for the entire game.

How did she find out? Why was the fight with the messenger even needed before she found out? Just to move "story time" forward and give her time to divine what is going on? The story just seems to stop making sense at this point.

I explored the void (and even obtained the worst ending) before fighting the fifth "messenger". Is this why the real messenger had nothing to say, or did authors really opt for cheap surge of omniscience here?