Lumia and Narvea's deals aren't implemented yet, but I'm pretty sure Keros says he ultimately wants to add it to the overarching collective soul the Kitsune have once Kasyrra is dealt with.
re: CoM
And I don't think we should be too worried about them eating our soul, either. My understanding is that the Wraiths devour souls because emotions/experiences taste good to them as soulless beings. To engage in some metaphysical speculation, maybe they ate enough emotions that they formed souls of their own from the resulting raw material? Alternately, there could be a "Gods Need Prayer Badly" situation going on and the Seven are sustained by the worship of their followers.
Whatever the answer is, the Living Gods possess souls of their own and therefore don't need to devour souls anymore. Probably.
Yeah, Because Keros is totally trustworthy and would never do something for shits and giggles.
After completing the convocation quest and meeting Mallach, It wouldn't shock me if they want it for more power to lord over the others or if some part of their old nature remains. Guess we'll have to wait until that part of the story is expanded on.