Why go to Slaanesh when the Changling could instead impersonate Boris asking for an assassination.
Why would the Changeling ask for an assassination?
It makes much more sense for the Changeling, disguised as Boris, to just launch into civil war against Vladimir and destroy Kislev that way.
And it would be much funnier for the Changeling to instead disguise themselves as Vladimir and then through poor rulership drive Boris into thinking his only solution is declare a coup against his father.
It is much more likely that if the Changeling is involved (and I don't think they are), they are actually disguised as the Tzar and their "incompetence" is actually a cunning plot to create a civil war, with Boris as the aggressor.