Ok going back to my idea for using the EIC to bring about a multi plan boon. Trade is incredibly useful and can help both Mathilde and Kislev. Weapons sells can help both the empire and Kislev. Just regular trade alone will help both sides.
Opening trade will help the empire, Kislev and Varg. We can bring the gong guild to Kislev. That is biter that can then be sold to Varg. Which then can be used to buy powder and sold to Kislev. Ok now before the communist block who hates Mathilde making money comes at me we can keep the margins low and pay taxes. Like what the EIC does in the empire. These will help Kislev with something it desperately needs, money.
Now the second part the EIC will buy books for the Grand Library. The library will pay the EIC to pay the Kislev institutions to copy their books and then have the EIC transport them back. These can be taxed but also helps the institutions in that they get money. Anyone who has worked for a library has experienced knows they constantly need money. Things break down, roofs need to be repaired, staff needs to be payed, paper/ink needs to be bought, and so on. Mathilde gets her books and gets eight peaks to pay for it while giving money to Kislev.
The third part is going to be tricky. Boris can not outright stop the ice witches from killing men who show magic. While some are allowed to flee to the empire a lot of the rural ones are just killed. The EIC can hire young men and boys as "guides" and if they happen to have magic while they are in the empire they can join the colleges. That is going to be a lot of political talk on Boris side.
Now the reason I propose to go about it these way is because Kislev needs money. Boris literally said Kislev is rich in people and resources but poor in money. Mathilde's gets books, men do not get killed for having magic(which I believe is something Mathilde would want since it is close to her), and Kislev gets money to help Boris repair.