This was my thought as well when I did the research. From the timeline, barring any "behind the scenes" actions that we didn't witness, Than seems to be completely overtaken by the time of Livia's attack. But, what troubles me is the fact he is wearing the necklace much much earlier. If this was some kind of "homing device" Lahabrea used to keep tabs on Than (and later use to teleport into the Waking Sands), how did Than not notice what he was wearing? He had to have known he had it. Maybe he bit off more than he could chew?

The Garleans and Lahabrea have been wanting to kill the player since the start, so even if Than was slightly overtaken by then, he would not have let the adventurer out of his sight..or would have warned him he was about to be overcome by the darkness. It is also strange that someone with a high intuition like Y'shtola didn't sense a thing o_O