You are correct with regard to the argument, but as I was reading this two words came to mind: Deniable Plausibility!
No paper trail, no communication, if you fail, your actions will be disavowed. (Mission Impossible). Only a theory, but....
Hissrad could have had a code word or action that would trigger the order to him to report in for further instructions. Until then, his orders would have been to watch and report what he saw. Like any good spy cell, he would have no idea of what any other unit was doing until there was a need to know. By the time he did know, it was too late to go, "boy did you guys screw up." All he could do then was suit up and hope to defeat the Inquisitor where they met. Or at least that's my working hypothesis.
You mean, plausible deniability? That's all fine and dandy, but if they really didn't know that Viddasala was doing all that, they would still deny any involvement and disavow her. So the onus is on the other side to prove they were involved or sanctioned her plans.
It's possible that Hissrad was trained to blank out and become a killing machine when Viddasala says a certain thing or writes it. I might even take that as canon, except for the fact that Hissrad still shows he's in his full wits by saying, "Change of plans. Nothing personal, bas." So he gives the Inquisitor fair warning before attacking, shows some concern over perceptions of himself (nothing personal/don't hate me for this), and then delivers a play on words when he calls his "boss" a "bas" instead. So he knew what he was doing, and my problem is still the fact that he should have known Viddasala was not his authority.
Also, Dragon's Breath had failed by that point, so disregarding the order was past the time when Hissrad would realize Viddasala screwed up. The option of him staying behind and then responding to Viddasala's letter or messenger is even worse. He easily could have destroyed the letter or killed the messenger and then put forth his own plausible deniability. I think he would have a better chance sticking with the Inquisitor and hoping to kill all of Viddasala's crew. I think Trespasser was trying to say that Hissrad didn't care about his own life, only following the demands of the Qun. The problem is that Viddasala was not the Qun. He was the Qun!