Getting the thread back on track instead of wild guessing that has 0 facts to back it up made by someone who seems hellbent on derailing every thread they post in:

It really boils down to the fact that somethings exists on the NeverNever side that Harry has been told will Kill Him Horribly.
We don't know much about Demonreach except that it's a prison for the worst of the worst. And that means the NeverNever side also has to be pretty darn dark and dangerous.

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The Great Beings don’t manifest or are very polite generally about doing so in the mortal world. The point about Ferro makes my point, the White Gods enforcer killed Syriothax and Michael could only have done so in the mortal world, ergo A Dragon was manifesting too much of itself in the mortal world and had to be punished, just like Ethnui.

The only being capable of enforcing such politeness is the White God, the most powerful being in the Multiverse.

The Knights ONLY do the bidding of the White God, that means principally oppose the Knights of the Blackened Denarius, but we have seen Michael and Butters take on other foes. We first see  both Michael and Butters taking on a threat to infants, that’s a pattern, the White God doesn’t like the supernatural attacking those who have yet to develop free will. That’s two rules.

1. Don’t enter the Mortal World if your presence is going to break reality and put mortalkind in danger (the Denarians were an attempt to get around this)
2. Don’t attack infants who have yet to develop free will;

If you are a Denarian or other breaker of rules 1 and 2 expect to be faced by the Swords and MY other Champions. I include The Queens and Ladies in the latter as well as everyone granted Soulfire like Harry, Odin and (probably Martha Liberty). The White God or at least Uriel look more and more to have had a hand in the creation of the Mantles, including Kringles

The White God is very Free Will.

The White God (the same God of  Michael, Butters and Saladin) wins hands down on number of believers, and has the other gods in a headlock.

The Knights don't do the White God's 'bidding'.
And again; literally NOTHING in the books has said the stronger beings don't manifest their powers due to some rule from the White God. Not once.
You are drawing conclusions that don't exist.
You are saying 'Michael killed a dragon + Michael is a Knight. Thus; the White God decided dragon's can't use their full power'. The conclusion has nothing factual to back it up over the more common 'The dragon was being evil, so Michael killed it'.

You will notice Ferrovax is not mentioned to kidnap and devour maidens. He's not nice by any stretch of the word, but he's not EVIL. Or, at least, not doing evil things during Harry's lifetime.
And since he's not being actively evil; he's not being slain by a Knight.

You are trying to make square pegs fit into round holes with your theory, as nothing in any material (that I know of) says the White God ever decreed 'Super powered beings can't use their power or my Knights will kill you'.
The Knights punish EVIL. Not 'Anyone who uses all their power'.
Your theory is wild guesses strung together by crumbs that have 0 connective thread other than what you try to hammer into place to make it work.

I don't remember in which book it's mentioned, but it basically boils down to someone telling Harry to NOT go into the NeverNever on the Island.
I think it was a villain, actually, taunting Harry about how he had no way to escape. Maybe Nicky-Nick & The Nickleheads?
Something I vaguely remember as 'I know what lives there. You're better off letting me cut your throat.'

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.

“Thou Shalt Not Open The Router Gates?”

I mean, lowering your firewall WOULD let Outsiders in and render your PC into a condition not unlike an Empty Night.

Idea: Christ's Crucifixion Was A Result Of, Or Action Against, The Outsiders

We know that items pertaining to Christ's crucifixion are back in play as weapons against the Outsiders. And belief holds that Christ gave his life to absolve humanity of their sins, right? Well, what if we changed 'absolve humanity of sin' to 'protect humanity from Outsiders'?
It would fit that God's son gave his life to protect humanity and, in so doing, created items that can be used explicitly to fight the Outsiders he was striking back against.
Heck, I would not be surprised if Judas was working with the Outsiders to kill Christ, possibly in a ritual aimed at God, but Christ turned it around and made a willing sacrifice to protect the world. I mean, why else would items of faith have power against Outsiders when there's not a whole lot of people believing they would?

This just popped into my head while reading other posts.

Harry baseballed Sith into the lake but he's THE Cat Sith. No way that killed him.
So, unless Harry will or has told Mab that Sith was taken, he can still infect other Sidhe..
And, I mean, harry knows Sith's power; he has to know Sith ain't dead. I'm hoping he told Mab between books, and gets cured, because he was a nice foil for Harry.