Taravangian would make Szeth work as a slave in his castle-cave-thing if he wanted Szeth to disappear. And I don't think his goal is to go and steal Alethkar behind the backs of the high princes (since it wouldn't work anyway... it's all nice and fine to go campaign for revenge, but the campaign's gonna end if your home is being taken).
and like I said, Taravangian would have had people following Szeth, since he'd lead to the oathstone. and since you brought it up, he'd also have people following the parshendi, since they had the oathstone.
From what I can figure, it's very very very improbable that Taravangian had anything to do with Szeth's part in the assassination. There's only a chance (I think the chance is very small) that he was part of the assassination at all. He more likely learned how the assassination went down later on.