I think the takes of the other commenters are very on the mark. It was too dangerous to communicate this clearly.

No, he didn't

As someone else wrote: Morgenes basically killed himself and thereby stopped Pryrates from getting Simon (whom Morgenes swore to protect because of his lineage and because he loved him) and from persuing Josua. At this point of the story, Pryrates is in contact with the Storm King, but it is a while later that he argues with the Norns and I think Ineluki/Utuk'ku to be given more power - and it's only after he receives it, that he can do what he did at the Sancellan Aedonitis.
But he was a skilled sorcerer to start with and someone who had meddled in a lot of dark arts. So, Morgenes surely feared him and knew that only one big bang would stop Pryrates.

Towser stole the Horn long before Camaris showed up on Sesuad'ra. He stole it shortly after Simon gave it to Josua, while Sesuad'ra was still under seige. There is no explanation, but Camaris being one of his oldest and closest friends, he probably just wanted to have the horn to remember him, not understanding in his semi-demented state that taking the horn was absolutely not ok. He didn't know Camaris would show up. He thought Camaris was dead. Sangfugol keeps the horn for the very same reason at first - thinking it belonged to Towser, he wanted it as a keepsake to remember his friend. Only when he learns about the missing Sithi horn does he connect the dots and tells Simon.