Couple of formatting errors.
So. My guess would be Skaven, trying to frame Marienburg - but I'm not confident in it. I think the residue implicates them more strongly than the other evidence points to other people. Acquiring Human goods is easier for Skaven than acquiring Skaven goods is for Humans.
Yeah, the biggest thing that suggests it's
not Marienberg is--honestly--the fact they used so little powder.
It suggests they either
tested it enough to get an idea of how much they needed for the job (Which suggests a surplus greater than the supply allows), or that they already knew how effective it would be (And thus suggests Skaven cats-paw given how jealously they guard their secrets)
And, to be entirely honest--the black market for salvage from Ubersreik is probably not conductive to getting large supplies of powder. The fact they
used this stuff suggests that the bomb-maker was familiar enough with Skaven Blackpowder to know that a hogshead would be enough to damage or sink a Dwarven Monitor, and Marienberg lacks a heavy Skaven presence if I'm not mistaken.
Yeah, Marienberg has
zero Skaven Presence. Not even so much as a minor outpost in that part of the Old World. It suggests they either managed to do bomb testing in the middle of Imperial Territory after somehow buying out the majority of the leaked supply and nobody noticed (A stretch given the current tensions), or the bombmaker was an active collaborator (Which is just bad news that justifies getting your ass coalitioned)
If they used a shitton of Warpstone-laced Black Powder and the effect was enormous--that'd be one thing. But this attack was carried out with surgical precision with exactly the amount of force necessary--that last part is the red flag because
this is not a substance that's readily available outside the Skaven.
The only source of it is--in effect--what scraps managed to slip past the Conspiracy of Silence after a rather hefty assault on a Skaven outpost. That'd be enough for someone to make a bomb, but it wouldn't really leave you enough to precisely figure out how how much boom it was worth.