Except that we do know that there are Qunari agents in and around Thedas, who obviously have been infilitrating the various nations long enough that Tallis had to protect their identities in MotA when they risked being compromised, long before Sten and the rest of the Beresaad were sent to Ferelden.
While it's possible that until the Fifth Blight the Qunari had absolutely no interest in Ferelden, seeing it as a quiet backwater compared to the Orlesians who have traditionally and historically being a lot more dangerous, the main reason that Sten was there as an advanced scout is because he was sent to find out information about the Blight. You wouldn't send a spy in to do that work, you'd send in some of the toughest guys you have, because you're expecting to find a ton of Darkspawn. If we assume the Qunari have had little-to-few agents in Ferelden thus far, then the chance to get additional intel on Ferelden defences and military in the process was a secondary objective at best.
I agree that the issue with why various Qunari interact with Mage Inquisitors, Wardens and Hawkes is problematic given their anti-Magic stance.
In the latter two cases, the Warden is likely defined as a Warden first and a Mage second, so the Qunari may either accept that they have enough discipline not to be corrupted or will leave that task upto the Wardens to deal with. Similarly, a Mage Hawke (post-Act 3) has a lot of Templar attention from those in power and Hawke's freedom is conditional on their service to Kirkwall, making Hawke just as "leashed" as a Saarebas, but with a lot more rope.
(Tallis meanwhile plays fast and loose with the Qun, so she's not the type to care that Hawke's a Mage. She may be a believer in the Qun, but she's not a very good Qunari when it comes to following orders, something that we know has gotten her in a lot of trouble in the past.)
As for the incident with Arvaraad, he may have been the only one in his Antaam, but the others were still subordinate to him. If he told a Ashaad to kill a Mage, even if that's not the job of a Scout, they'd do so because Qunari don't question their orders.
So to answer your question... I don't know. If Iron Bull was reporting on a Mage Inquisitor and they told him to kill them, I don't know whether or not he'd go through with the order or not? I personally don't think they would give that order despite the sheer amount of demonic contact that the Inquisitor has, due to their being the only one who can stop the Fade Tears, making them incredible vital to prevent catastrophe.
If they did give that order, it would likely be conditional to be carried out only after the Breach is dealt with, but until then the Iron Bull is to protect the Inquisitor with his life. The Qunari aren't stupid enough to waste an asset after all, at least until it's no longer needed.