In the "Blood of the Empire" comic it ends with Thanaton attempting to make a deal with the Dark Counctil to make him a DC member and grant him the title of Darth. Given the events in game, they did give him the Title of Darth, but refused to make him a DC member. He still had to earn that. Since those events some 20-30yrs back before the start of the game, Thanaton took his time to learn force rituals, gain knowledge and build a power base.
Thanaton is by no means weak. He may have been one of the stronger members who have ascended to the Dark Council. However his arrogance was his undoing. He believed his position meant he could do what he wanted without consequence and that the other DC members would Have to support his decisions. But really, when you start crap on your own, the DC will leave you out to dry.
As for Darth Jadus, we haven't really seen all he can do and it's more than likely we'll see more of him in the 1st expansion of the game.
No, Thanaton was made a Darth in the comic, he didn't become council member until the SI story of Belsavis.