And don't forget that the veil had been intentionally weakened by Mavra. That would give them more power.
As a tangential topic, I don't know why (probably because I'm dumb), but I never connected Mavra to previous feats of necromancy before. Maybe it's because I tend to gloss over the first three books, but Mavra and Cowl were both in town during Grave Peril, when a whole bunch of necromancy was going on (specifically Corpsetaker's brand of necromancy, with ghosts and specters and such). Could this be when Cowl and Mavra both found out about the Word being spotted? Is that one of the reasons Cowl was there? Or...did one of them arrange for Bony Tony to find the storage unit? We're talking about a mere four or five years before Dead Beat, so I find it likely that Tony spent a bit of time lining things up and slowly liquidating what he found.
As yet another side note... did Cowl specifically bring up Bianca's assistant's ghost to drive her insane? Did... did he basically engineer the circumstances that kicked off the war?
Sorry, it's a bit of a tangent, but I find it interesting that Mavra and Cowl were both hanging around both times necromancy was used.