lol whats with the attitude? It's not that complicated. Morathi comes back and is the perfect hiding place for Slaanesh since she's been his most devoted acolyte since she first popped up and is pretty much his avatar of corruption being responsible for inserting that wedge into the Elves in the first place. But why wouldn't Slaanesh, a god where one of his primary attributes is corrupting personal experience, want a more hands on role in that experience and playing those political games? So Morathi mysteriously is still around but somehow changed happily skips along with Malerion to join Sigmars grand alliance, the perfect place for him to work his own corruption right at the heart of the realm that has been locked off from his brothers. This is until Tzeentch decided to tip off Malarion and Tyrion at which point they threw a bag over her head and ran off wondering what to do with her.
You seem to have missed the whole point that she's the disguise, the hidden lair. They're supposed to notice her since she's been there all along. It's what was hidden inside her that they didn't know was there. It's probably not the case but it's totally possible.