Depends.
Just removing it? Not very. You may not even need a Working and just use the Path.
The issue is that the nails became mystically imposed on the Primarch's soul. It became such a fundamental part of Angron's mythos that even becoming a daemon prince did not remove it. Heck even Sanguineous ripped it out of him at the Eternity Gates, when Angron respawned, it came back.
So to truly remove it and restore Angron? Celestial Working minimum.
Would the fact that Angron was successfully resisting the influence of the Nails up until the Emperor got all his loved ones killed to prove a point make it less hard, assuming we manage to keep that crisis event from happening (possible since it takes 42 years to find Angron after Lorgar)?
Conrad Curze is a mad prophet that grew up in a shit hole the only way you can use magic to fix him is the same way you could use mind control to fix him. The Night Hunter specifically went out of his way to choose the path he walked and denied any that might walk against that path.
Counterpoint, Konrad isn't discovered until 39 years after Lorgar, so if we can find him faster, we might be able to catch up with him before he goes fully off the deep end. We almost certainly won't be able to parent him but we might be able to mitigate the damage. Especially since we can probably teach him to better control his visions.
Therapy. Lots of Therapy.
Necron FLT is also very slow in intersteller distances, nowhere near as fast as Warp travel. That was why they broke into the Webway.
There are no "lesser spirits" only daemons. You would need to raid a Covenant black site and get a book of names if you want to summon them. Though keep in mind that right now, the Covenant is positive to the point that they kicked out Kor Pharon for being too Zelous in converting people to the faith. So there would not be a lot of that going around.
That will change later once you get started.
Yeah, I know that but we IC don't know. I was trying to indicate that I'd want to start with a Fury or maybe a Nurgling, as opposed to a Herald or Keeper of Secrets. On the subject of Necron FTL, because of how it works it's actually faster over longer distances, though it is comparatively slow over shorter ones if we ignore the inherent unreliability of Warp travel and the advantages "none of our ships got eaten or didn't show up for 500 years because the warp was uncooperative" grants such empire building concerns as logistics.
Digression time:
Gods in exalted are a function of the world or rather the beings and parts of the world they arise when things are created or remade or reformed or they are made whole cloth by greater gods or greater exalted or the primordials. The gods in exalted are a mix between Shinto Chinese Heavenly bureaucracy and Greek they're little nearly non sentient things to greater than human in both thought processes and Powers.
Expanding on this, the primary reason the Primordials made gods is because Creation doesn't have a natural Veil. The Wyld (which, as previously mentioned can be thought of as analogous to the Warp) is just out there if you walk far enough. Continuing to exist then requires an active exertion of Will, and making things play by consistent rules is an even more complicated endeavor. The Primordials, when creating a Glorious Cosmic Xbox decided to resolve that issue by making gods. Everything in Creation from the smallest grain of sand to the tallest mountain has a god whose job it is to make sure things keep working the way they're supposed to instead of getting eaten by the chaotic influence of the Wyld. The vast majority of these gods are Least Gods, incredibly weak, a bit dumber (and significantly slower) than a dog, their only job is to just sit on their personal blade of grass and go "yes, this is a blade of grass. It is green and needs sunlight. It can be eaten by cows and it definitely exists." Some gods are gods of bigger or more abstract things like a mountain or a road or the concept of artificial flight and their job is mainly to manage the least gods, make sure no one without the right paperwork starts twisting the laws of nature and generally maintain their domain on a macro scale. Of course with their greater power and intellect, these gods were pretty quick to invent politics and slacking.
