The closest parts of the NeverNever to the mortal world is Faerie. That’s why most of the Ways travel through it.
It’s a WOJ about the Previous Defenders but can be deduced from whose power they made the Mantles of Faerie the current Defenders.
I believe the White God has issued a proscription on god Level beings being active in the mortal world, the Queens are the most powerful tolerated, (and that I think was designed into the Mantles) the more powerful Mothers stay in the NeverNever, along with the likes of Hades, in their own realms, very deep within the NeverNever difficult for Wizards to reach.
Some gods took a demotion like Odin to stay in the mortal world, great Dragons like Mr Ferro manifest only part of their power. Ethnui was in breach of that Proscription, and ended up having two Knights and the White God’s Favourite Idiot sicced on her, leaving her in the one place in the Mortal World the White God accepts her presence, Demonreach.
As Mr Ferro said his full presence in the mortal world would break it, which is why the god level beings are not allowed there.
The only reason for this is to allow humanity to evolve, they will be the next Defenders, and given Jim’s age and reading habits humanity when it builds up its numbers and technology will create the Singularity AKA the White God.
Nothing says there's a rule against great beings manifesting. Certainly none made by the White God.
The reason Ferro does not manifest is because:
1. It would anger humanity, and if 1 Knight can kill a dragon, then all 8 billion people, scared and angry at the last freaking dragon levelling a city can do it.
2. The one time his full power is needed (The Battle For Chicago); he has to hold back or he'd level more of the city than he'd save. Meaning manifesting would run against his goals.
At no time does Ferro says he cannot, or that anything would stop him.
In Battle Grounds, he says appearing in his true form would harm the city, which no one wants. Not 'The White God prevents me' or 'It would anger the White God'. Nor does he imply that he can change explicitly because the White God is letting him.
And nothing implies that the White God is punishing Eithnu for using too much power or breaking any of His rules against such.
The Knights take to the field for the same reason they always do; to stop evil. Eithnu was killing scores of mortals, which is the same reason the Knights fight the Nickleheads; stop stop evil (and to save the possessed).
At no point have the books ever so much as hinted that little-g gods were limited by the White God.
In fact, what few little-g gods we have met tend to say things like 'My followers are few and scattered' (Odin), implying it's BELIEF that gives them strength, or takes it away. Not the White God.
As people stopped believing in Odin (as in actual followers who prayed to him), he lost much of his power. He as much as says so when Harry asks for help in Changes.
I think it's something like 'My power is not what it once was. My followers are few and scattered.' Not 'The White God (or anything else) prevents me.'
Unless I am really missing some parts; nothing has said the White God is not allowing them to have full power.