It was just a theory to explain why Dakar might be incredibly pissed of with Arithon. Dakar could still be injured or held prisoner by someone, even Koriani, we will have to wait and see.
If you read through the books, Janny has left clues to places Arithon is going to be visiting at some point in the next 3 books. And since Arithon is the only mortal who can redeem the wraiths and not even the sorcerors would have the cheek to incarcerate Arithon in Rockfell (not after the Marak business) to achieve that it seems likely he will have to break in. Most of the sorcerers are not going to be encouraging Arithon to do something as dangerous as risking freeing the wraiths, some might even try and stop him. I doubt Arithon is suddenly going to feel like correcting all the mistakes of dragons and humanity that threaten the Paravian survival and the compact, at least not yet. But to save Traithe would not Arithon be willing to try to redeem the wraiths at Rockfell and see if there was anything else being held imprisoned there.
As the hints, the sneak peaks (of which there were 5) are not the only place Janny gave us info about IT. This thread started way back in August 2008, for those who want to read through the entire thread Janny gave out little bits of info as she was writing. Not much, but a bit.
11 pages, that was quick.