contentment is a better translation to english in this passage, i think, than content. so like, happiness/fulfillment/satisfaction/actualization, i think could all be synonyms substituted in its place here.

all in all, means, that God is just himself, and cannot be defined or constituted by things of this material world. so you can't define god, in terms of a human. So the monotheist's God is not motivated my material concerns, and so why should he care if a human is "empty"? Why would God care? It's a critique of the standard religious dogma about atheists and nihilists and unbelievers being "empty of purpose" and the other usual tired insults they use to bully people into their authoritarian conformity and hierarchies.

Creative nothing refers to the capacity for negation and rejection that creates the conditions of possibility for new things. Like, an attack on riot cops creating the possibility for a rave to continue. In many contexts, you have to have creative nothing, in order to have anything but misery.

This is a rejection of the christian attack on atheism's lack of authoritative source for morality the way that monotheistic religion is. It's a defense and celebration of "emptiness" of ideology and dogma, i think in my analysis/opinion.

So I haven't read Stirner but I've had enough people explain it to me that I can take a shot at this, not being sure whether it's right or wrong.

By "content" here would be meant "what it is" - the idea that there is some "thing" that "God" is, or that "humanity" is, independently of our minds. To have "enough content in themselves to be all in all to themselves" seems to me to mean that they are real things outside of us with a kind of fixed and distinct being, rather than a contingent construct (spooks/spectres) emerging from the interplay of material and historical processes.

"All in all to themselves" seems like a reference to their outsideness as concrete things, then. So, to rewrite the first sentence I would say something like:

"If God, if humanity, as you affirm, have enough content in themselves to be all in all to themselves,

"If god, if humanity, actually had being on their own,

then I feel that I would lack it even less, and that I would have no complaint to make about my “emptiness.”

then that being would be even less a determiner of me, and I would have no complaint about how they [those external things] are not determiners of me."

The creative nothing is a unique, a life that is not unintentionally mediated by "spectres". This is not necessarily a person as we understand them, as a subject. It's more like a collection of life in a state of pure becoming.

All things are nothing to an egoist (when in anarchy, because full egoism requires the material state of anarchy) because the egoist's life is not determined by fixed abstractions external to them. In that sense the egoist is nothing, is made up of nothing imposed from the outside - they are rather a pure potential, a becoming, rather than a being. And so, everything that an egoist has is their own, because they are not owned first by the abstractions, but use them as they see fit for their own purposes.

I like to conceive of this somewhat simplistically with “content” representing meaning and substance. God and humanity are totalities fully formed and conceived (“all in all”)and therefore closed.

You, on the other hand, are void of such content and so a “creative nothing.” If you start from emptiness your possibilities for creating yourself are endless.