Ok, let's do this backwards first, who would be the least useful with dealing with forest spirits (we think)
[ ] League of Ostermark
The trade league of Ostermark's nobility are likely to see you as a shareholder of the EIC first and a Grey Wizard second, but they more than anyone have a vested interest in peace and prosperity reigning over the Upper Talabec
Absolutely not their wheel house.
[ ] Morrite Witch Hunters
While the citizens of every province of the Empire respect Morr, Ostermarkers revere him more than most, which means that Morrite organizations like the Order of the Shroud, the Dreamwalkers, and the Andanti fill the role of Witch Hunters in Ostermark where normally the Templars of Sigmar would hold sway. Get in touch with them and see what they might know.
It important to note that morrites do have ways to deal with sprints, not just ghost…. Its just that, unlike ghost, those ways boil down to 'stab it'.
[ ] Elector Count Wolfram Hertwig
Chancellor Wolfram Hertwig has a reputation for being sensible, and if anyone would know what's going on in Ostermark, it would be him. But approaching him openly would make your presence here very official, which might get in the way.
I won't say useless because he would be able to make all the other groups on the empire side jump to it. But I'm not sure about escalating to the big guns yet.
[ ] Tsarevich Boris Bokha
You've met him before, and he seems to have made it his business to solve problems that his more straightforward father can't or won't. Seek him out once more and see if he's interested in solving a source of unrest and disruption in the Southern Oblast.
The same story with the electer count, but because he could take to the hag witch's and make them jump, he is arguably more useful.
So those are the ones I think that are out of the running while the theory is (forest spirts)