I think he is genuine also killing one of our very few wizards even though he hasn't done anything yet is not a good idea.
Dude, considering that he's thinking of even
touching Skaven weaponry/artillery, which is
filled with Warpstone IIRC, should be sending warning bells everywhere. Wizards and Warpstone(Hell, anyone and Warpstone) aren't a good combo. Plus, he wants to do it in the middle of any already risky reclamation of a Hold, thereby making it even
riskier and that means bigger consequences if/when he fucks up. And that reflects upon
us since we took up the burden of leadership.
As for killing him, it neatly solves any potential problems, and we have the perfect excuse in this reclamation of the Hold(Oh, poor Johann. He unfortunately died via Skaven/Orcs/blew himself up using the Wind wrong/whatever).
And if you don't want him to die, then fine. Knock him out for the duration of the reclamation using some poison and then deliver him back to the Gold Order with a warning that he's been researching the Skaven and warpstone and that the Grey Order is now keeping an eye on him or that the Gold Order can deal with him however they like. Or just knock him out, use our Shadowsteed to travel in some random direction X leagues away, and drop him off, leaving him to fend for himself and make sure he can't fuck up the reclamation trying to mess around with Skaven/Warpstone bullshit.
Point is, he's shady as hell, Mathilde still can't trust him/isn't sure to trust him, we're not in a spot where a mistake is acceptable/containable, we're in an already risky situation of trying to retake a Hold, and
he's trying to fuck with Warpstone. I, for one, don't want him sniffing the fumes/getting a contact high and then turning into some kind of warpstone abomination and causing trouble for us all.
Look, if he really had any good intentions, then he wouldn't have come here, he'd have gone out into the Wastes/Under-Empire/wherever the fuck* Skaven can be found away from the Empire and its allies, where if anything goes wrong it won't be around friendlies(ie; us and the Dawi who wouldn't take kindly to a manling trying to reach beyond his grasp, especially a
wizard manling, which could get a Grudge against the Gold Order/fuck up relations between the Empire and the Dawi), (and died) quietly trying to find out the secret(s) of Skaven artillery(Or in the unlikely even he succeeded and was being truthful, can come right back and share the knowledge). Or he'd have several others of his Order with him who are following him/a writ from the GO and thus can truly say that the Gold Order is backing this, not some lone wizard going off the deep end either fucking with things he doesn't understand(or trying to steal tech from allies).
*Hell, why not Cathay?