In this past weeks episode ‘Warhead’at the end Thrawn doesn’t hesitate to inform Kallus about their victory despite the initial loss and doesn’t make any comments alluding to the possibility of Kallus as the traitor. Though we the audience think Kallus is being a little too obvious, that might not be the case for the on screen villains. Or course it is possible Thrawn is trying to make Kallus more relaxed so he’ll slip up but I’m not one hundred percent sure anymore.
Yes. I do suspect him.