Ishar didn't get his Honorblade until a few months before RoW.
Well, she can lie, appear to Gavilar, and see the future. But I don't think she has a tie into the Oathpact, Unless Honor added her, nor do I think, judging based on Odium, that she could end the Visions.
That being said, she presents one of if not the likeliest option for an imposter.