So it's actually not Michael, but Father Forthill who reveals this information.
Also, while Anduriel can hear and sometimes see out of any shadow, he can't do this out of every shadow. I highly doubt he can do it on consecrated ground like that of a church, given how hard it is for spiritual entities to even get onto church ground. Talk about a threshold.
....
I don't mind the Kim Delaney theory. Definitely has some merit. What's Kim's connection to Cowl though? And why Kim over Harry (the strongest wizard in the city) or Mort or anyone else? What's Kim's connection to the dark side?
Plus, the blessed kerchiefs the Knights wrap dropped coins do in seem to be able to block the summoning. And that's just a portable field measure to secure them for transport, so it stands to reason that their coin jail monasteries have better countermeasures. On the other hand, it's also possible they're mistaken about it being the kerchiefs (they do get a fair bit wrong about the Lash situation) and it's actually the presence of the Knights themselves, or near-saintly priests like Forthill, who are jamming the summoning like they jam Anduriel's listening. Rotate the wrong dude out of the monastery, and the merely averagely-good personnel no longer pose a problem to Nic and Tessa rubber-banding their crews back to them?
As for the Kim theory, she doesn't necessarily have to have a particular affinity for the dark. Kumori seemed more like a deluded idealist, with a side order of healthy fear of how easily Cowl could kill her if she accepted Harry's suggestion to turn on him. Telling her that he could reverse her necromantic resurrection - or convert her into a regular controlled zombie if she got out of line - would be a pretty effective stick alongside the carrot of learning to use dark power for her own morally good (at least, self-proclaimedly so) actions.
The main drawback I see to it is that the kind of necromantic resurrection she demonstrated with the critically injured gangster in Dead Beat seems to require the deceased to be pretty fresh. Death being a process rather than a binary, it seems like necro-healing can only catch them up to a certain level of how far gone they are. So if Cowl had snapped her up as an apprentice acquisition of opportunity after MacFinn killed her, the window of opportunity is almost certainly before she made it to burial. And while I'm sure he could do that easily in and of itself, it would leave a missing body where the official system is expecting one to be accounted for. Given Karrin's interest in her, that seems like the sort of thing it would be a stretch for her not to notice and end up questioning Harry over. Not impossible if he was using mind magic really widely to cover his tracks, but still a pretty big stretch.
I will give credit that most of the Kumori theories have been blind to the possibility of Cowl bringing back a dead character, though. That bears considering.