I have to disagree with this. Nale talks about Tashi in the present tense. I don't think Nale knows that Taln is back, because, well, that would mean the Desolation was here.
This seems to me to be a much more solid point against Tashi = Ishar. Though I can see Nale respecting Ishar's expertise, while acknowledging that his personality has become a bit unstable. A "He's a genius, smartest storming man I know, but he's not so good with people, if you know what I mean" situation.