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.
Part of the danger of Anduriel is apart from the Angels themselves, I doubt many beings know the full extent of his powers. Perhaps not even most angels necessarily (he wouldn't be much of a spy master if he couldn't spy on his own kind). So it wouldn't surprise me to learn the Church is unaware of certain loopholes. Not to mention I suspect the Church is riddled with misinformation (and indeed, disinformation perhaps from Denarians etc).
Kim just doesn't seem like the type to touch dark magic. By connection, I mean she had no link to the bad guys. No sob story to make her turn dark. Apart from being murdered, I suppose.
Kumori, as you point out, seems deluded. Maybe even magically so (enthrallment comes to mind).
I agree, the longer the being is dead the harder it is (particularly if it is human). Probably to do with things like souls having their final resting place etc. But I wonder if there is a hard limit for how long a person can be dead for. I suspect not, but it might just require special circumstances etc.
In any case, why would Cowl expend such resources on a throwaway hedge witch? She was no solid power herself. All to upset Harry? That would say Cowl has a personal interest in hurting Harry, and yet supposedly the two are unconnected. If Cowl personally wants to hurt Harry it would suggest he and Harry have history, which makes other theories far more likely.
