SuperSonicSound
Shadowmancer of the Shadow Cabal
Ah, okay, if Marienburg is automatically suspect that it is most likely a frame job.
However I don't really see much evidence for Skaven involvement. The Warp powder may just have come from unscrupulos or desperate people going through the remains of the Skaven undercity in Ubersreik. The evidence is not conclusive enough for an accusation, but I suspect that someone in the Empire is trying to lure the Dwarfs into open war with Marienburg, so that the Empire can annex it.
Making warpstone gunpowder that actually adds to the boom rather than just making it poisonious is something that would absolutely require expert knowledge. I don't think you could expect desperate people to simply grind warpstone down and add it to gunpowder and get the effect they want, not with out a massive and drastic jump in the number of people mutating locally. aerosolizing warpstone via grinding it down to use as an additive is what makes it so unlikely that it was a human force.