First off, that's a great post, Immy. I actually cheered when Raefindan tackled Aeron. Quite in character. Nice writing.
Good questions, everybody. Mellonin and Gwyllion are not in the same place. Neither of them have been poisoned. Rather, they've been drugged for long lasting sleep (easier to keep them quiet and captive that way), and such a state makes it easy for him to torture them with nightmares.
As for Gond and Erundil, they have been poisoned. Tharonwe's purpose with this is to intimidate those who remain free into becoming compliant. The poison is not fatal, but the party don't know that. It will render those poisoned sick with high fever and shakes, and inability to keep any nourishment down. Tharonwe has the only cure. He will be contacting Raefindan and company in the morning with his demands.
Finally, Tharonwe has no intention of giving back any hostages, not until he has gotten his way regarding Amroth. He does not realize that Amroth wears the flesh of Mellonin, but will figure that out eventually. Tharonwe also intends to do more capturing, especially since it has been quite successful so far. More than that we'll have to see as the story unfolds.