If you think that the forest is too dangerous for the Lord Magister of the Grey College to scout, then you're saying that it's unscoutable period.
Mathilde is very, very good at stealth, and at survival, and she has an entire country outside of the spooky forest to back off into. I'm not saying she should go 1v1 Drycha, or indeed go do any kind of assassination/sabotage at all. I'm saying that I think the risk to Mathilde's life is small enough, and the gains from piercing the information veil are high enough, that scouting is the most valuable thing we can provide right now.
On the first point: She's never done stealth in the woods. A dedicated woodsman (Taalite priest if possible, but not necessary) would be both stealthier and more able to gather useful information based on what he sees. But yes, the spooky woods filled with asshole wood elves and extremely asshole dryad waiting to spring a trap (and therefore making sure no one gets close to investigate said trap) is pretty damn close to unscoutable. Especially when the scout in question is Not-A-Woodsman-Mathilde. We didn't try to sneak into the depths of the Eshin base until they were
very busy, and in that case Mathilde had time to prepare and they weren't waiting to spring a trap.
As for saying the risk is worth it? There's a few points on that.
First is that, frankly, I don't really give a fuck about the risk/reward ratio. I don't want Mathilde to die on a (already completed) sidequest. I just don't really care that much about the people at risk here. If Belegar was leading a throng, I'd be more inclined. I like the Kislev guys, and they're right proper badasses, but I don't like them that much.
Second, it's really not clear that scouting is even the best option. Getting some infantry to fight in the forest is the first best thing. So scouting is at most second order. But what can we expect there? I
could be really important information. Or Mathilde could fail to get close enough, or only find that yup, theses guys sure are planning a trap. That is absolutly critical information, but because the scrying worked well and Mathilde told the locals, that was already deduced. So the next bit of really important information is "Are they doing evil magic things?". And scouting might show that. But there's also a low risk option to check that, and the option to bring in a bunch of very grump magic grandmas*, which would help with that as well. I'd rather do one of those.
*It's my opinion that all ice witches, while unobserved, are eastern grandmas. It's a quantum thing.