Nicodemus wouldn't appear in dead beat to do a darkhallow even if the information was given to him on a silver plate. Anduriel would not like it.
I'm sure Anduriel wouldn't like his host getting that powerful, true - but I don't think someone without inherent use of magic could do enough necromancy to work the Darkhallow, either.
You mean what delusion Anduriel sold him? Do you really think it is that important
Anduriel might manipulate him about tactics-level decisions for expediency and to shape an even more corrupted host (like 'kill your daughter for the sake of the mission') but I don't think it follows that Nic is duped about the broad cause they're working toward.
I think from Nic and Deirdre's dialogues, the general outline of their thinking is pretty much analogous to how Cowl rationalized pursuing the Darkhallow (presumably because Kumori was within earshot) - the established order is so fragile SOMEBODY is going to succeed in overturning it, so it might as well be the less bad option. I think the Denarians who willingly cooperate with their Fallen believe Hell's apocalypse stands the best chance of averting the even worse Outsider version, so they're gathering power for that cause. The Archive's functions in coordinating the Oblivion War could easily have been adapted to tracking down Outsider cultists for assassination, and Hades was quite open about confirming the artifacts were weapons.
The plague plot in DM might even have been connected somehow - perhaps culling back the mortal population would have hurt their rivals (fewer summoners for Outsiders, less food for the Red Court, fewer practitioners for the Fomor to kidnap for their plans) in addition to the vague description that the Denarians get stronger by spreading suffering.