Thematically appropriate, but mechanically kind of iffy. The effects of Thresholds have never been properly defined.
Well, not all-powerful. (EDIT: Just to the point that mortals, even say the Merlin, might as well not try. Mother Summer or Winter could still slap her down.)
And I don't think it's actually purely based on being in a place of power, though that's important. The place of power = I'm stronger thing seems to apply to powerful-but-more-fightable entities like naagloshii and the Lords of Outer Night, too (and arguably Harry, now, though probably not the same mechanism).
Being in the mortal world does per WoJ make them more vulnerable... but that's slightly different.
...
Not as far as I know. I just don't see any evidence/mention that they ever are bodiless. (Though that's kind of an elusive concept if you're in the Nevernever anyway... even pure spirits and ghosts and stuff seem to be physically-interactable there ... and Bob has a sort of very limited 'body' even in the mortal world when he leaves the skull...)
Thinking over it a bit more, I think I'm gonna do this your way. I still think gods might be bodiless (like ghosts) just going by the general impression I've received from them, but...mechanically speaking, it's much easier to do this your way.
I'm thinking I'll rewrite A God Am I so that it gives some benefits while in the Nevernever and some while in your domain.
In your domain I'm thinking it can give you +5 miraculous power, +1 to all rolls, immunity to attack, and the ability to target the entire universe with miracles. Maybe also ACaEBM or something like it.
In the Nevernever in general, I'm thinking it can give you +2 miraculous power and some extra durability.
Sound like a plan?
I'm undecided about how to handle a Knight of the Cross or a stronger god challenging a god in their domain, so suggestions for that would be appreciated.