The more I look at the Stacking Advantages plan idea to just get to know the Frost Ward while also statring to lay the foundations, the worse an idea it seems
Mathilde: Hey Cadeath, how are things going over here, mind showing me around?
Cadaeth: Sure, let me introduce you to some people
....
Mathilde: Well thanks for the tour, be seeing you.
Cadaeth: Not a problem, say about when do you expect to start up the project?
Mathilde: Oh, I've been working on that for the past few months, making great progress.
Cadaeth: I see... I suppose our invitation got lost in the mail...?
The fact that there are so many issues to discuss and that it's such a controversial topic are indicators in and of themselves. There are just too many things that could go wrong, up to a fringe chance that he's not a cultist yet and a critical failure here pushes him over the edge. Why risk that?
Why did we hire him then, since he could be a cultist afterall? Sorry I don't find that reasoning particularly compelling. You weigh the balance of evidence to decide risk rewards factors. And the balance of evidence from Egrimm in the various scenes he's been in is pretty clear, he has no issue informing on Alric's movements to us, he's done it, he has no issue wishing and hoping for Alric's failure, he's done it, he has no issue hoping for Mira's success, he's done it. At some point you just have to look at what's been shown and accept it for what it is. Rather than hedging against remote longshots, or else you're not going to get much done. So I find the risk to be negligible, while the reward is an incredible resource to plumb regarding Alric's history and thought processes.
"Creating a cover and inserting himself into a population"? He was a Gold wizard, openly declaring himself to be a Gold wizard, joining an expedition that included wizards. He was lying about his eventual goals and about his exact rank in the Colleges, that was the extent of his deception. How is this anywhere close to doing detective work?
Yes, he successfully lied to everyone and got them to respect and accept him (Rather impressive considering dwarves are not noted for being particularly quick to warm up to people) while maneuvering himself into a position he could pursue his actual goals. Exactly the kind of social engineering skills that would serve him well in chasing down leads.
Finding things out from people is not an incredibly niche skillset at the end of the day. The skills he has map well to it. You don't need some hyperspecialized skillset to get people to think you have a good reason for wanting to know something while also making them inclined to tell you that thing.