At searching for anti-order forces? No, we aren't one of the best Greys. We are good at a lot of stuff, but uncovering secret cults isn't what we are exceptional at. We are very good at it, not exceptional. Look at our skills: which of them is relevant? Interrogation and our knowledge of Chaos, that's about it. We gave the wrong (useful, but wrong) recommendation re: skull river, had to be told about the Nordlanders fleeing, and I could go on. Working with Dwarves has meant we haven't needed to be good at this also. We are great at being secret, not so much grabbing other's secrets. And by not so much, I mean we are pretty good at it, just not excellent.
Sadly, we are the only option to investigate prior to an assassination, because we can't employ the guys really excellent at it. But we don't even know that a good check of Boris has been done.
... Sure, so we might like to search for a smoking gun? Not much of an ask here.
In contrast, you want to take his word at 3 meetings when we have know knowledge of institutional checks that would have happened.
... Again, you are doing a fallacy of the excluded middle. There's a level of investigation that we can do. We know the Empire is better at investigating than us, so we are fine with the Emperor. We don't know that Kislev is. So we investigate. Not hard.
First off don't Spaghetti.
Anyway, that is still more than sufficient to deal with anything other than a very high tier magus, and when did we learn that we were wrong about skull river? We know a Marienberg place burned down cause they were chaos, not that they were the ones behind it. Why would we learn about the Nordlanders fleeing from just being able to glare at someone, its a bit different between that and seeing that they're hidding mutations under cloths or surpressing their gift of magic.
We can assume that the people in Kislev where chaos is a much bigger threat on a day-to-day basis and someone who regularly interacts with Hag Witches and Ice Witches has been checked.
Yes I am willing to take him at his word because he has done the same thing for us. The same way I am willing to assume that Kislev has at least equal if not better means of detecting chaos cultists than the empire does, especially in the aftermath of Kattrin the bloody and that he interacts with them regularly.
And that's not even what that fallacy is!
My premise is
1) From our best observation Boris Bokhah is not behaving unusually compared to all our prior interactions with him which would indicate he's a servant of the ruinous powers.
2) To our knowledge Boris Bokhah has good faith reasons to want to do this which we can easily understand.
3) We are aware that Boris Bokhah has equal reasons to be highly suspicious of us that are at least equal to what we have to be suspicious of him.
4) We can reasonably suspect that Boris Bokhah has been exposed to at least as strenuous vetting procedures as we'd see in the empire.
5) Therefore Boris Bokhah is what he appears to be.
To be engaged in the fallacy I'd have to be leaving out something that would change the result. Like "Boris Bokhah had an unexplained disappearance and came back months later without explanation."
If this ultimately boils down to "do you think it reasonable that he wants to assassinate his father" then that's just a fundamental disagreement on the premise. Based on what I've seen of the man, yes I think it reasonable that he would come to that conclusion when pushed by circumstances beyond his control as a final option.