Demonreach has a huge summoning circle, so it can drag a potential prisoner there if Harry has a true name or blood to make the connection and not all the senior council know Harry’s relation with Demonreach, Eb and Listens know for certain, the others may know of Demonreach’s true purpose but not realise Harry took it as a Sanctum.
Christos doesn’t have the clout without the Merlin to get Harry thrown out, Harry has consistently undercut the Merlin’s authority, ended the war with the Ramps in utter finality overshadowing the Merlin’s counter attack and has custody of Excalibur, that’s got to sting. Christos may have gone Wormtongue on the Merlin to encourage it.
Nick wanted the spearhead, he didn’t want the rest of the items but to use them properly together as a ritual would likely require soul fire, why? Because Harry has been given it, its a Checkov’s gun