We do seem to be having a couple of gaps here. I'm not asserting the need for direct or conscious control. And you seem to be asserting that once a thing becomes real it continues to exist, but I would argue that something whose existence is contingent on it being continually imagined to exist is a figment of imagination manifest in reality.
Even if I agree with you here, the "manifest in reality" part makes a difference. Let me quote you out of sequence:
On a more real,world example, consider the idea of a nation. Nations are products of shared human imagination. They do no actions, since in reality all their acts are really collective human (and tool) actions. But the nation as object of multiple imaginations is given a personality and attitudes and responsibility for actions taken. It is both useful and dangerous to deem that it exists outside of imagination, but does it?
A nation is never "manifest in reality". It's a phenomenon, but not a thing. Gods in such stories are different: they're
there. You can throw rocks at them (or if they're too small, you can throw them at rocks). That has implications:
1. An object that manifests in reality must contain more detail than we imagine. Imagination always stays incomplete in some way or another; real things don't have the luxury.
2. Meeting an object (even if it were exactly the same) you merely imagine has an effect on your imagination; e.g. you might be focussing on "hidden" sides now that you've seen the real thing.
3. (1) and (2) are only about the imagination of one person. But if we're dealing with more than one person, there might be contradictions in how the object is imagined. As long as the object doesn't manifest, this is merely a problem of imagination. Not so if you actually concentrate all that imagination into a single formed space-time co-ordinate.
How you deal with these problems determines whether the trope comes across as anti-theist or not. The trope itself, IMO, is not
inherently anti-theist.
(1) and (3) suggest that there's something that concretises overlooked detail, or engages in pick-and-mix activity from different imaginations. It might be an abstract principle, it might be the instinct of a formless sentience...
(2) suggests that interaction grants a sense of reality all by itself, in the sense that you can't imagine what you actually see. The philosophical implications run deep, here, all the way to "All the world's a dream."
So on to Pratchett:
In Terry Pratchett's Hogfather, a group of entities hire an assassin to kill the Discworld's equivalent of Santa Claus. In order to do this the assassin uses rather amusing but spoilerific means of manipulating the beliefs of children so they no longer believe in The Hogfather, as a result he ceases to exist. The MCs in the book work to undo this in two directions: one by stopping the manipulation and two by faking evidence for the existence of the Hogfather. The children who bring this about do not have conscious control of the god, but he exists as they imagine him to be and acts as they imagine he does.
In this instance, I'd actually agree with you. Pratchett has always struck me as a humanist, though, not only in the way he treats gods, but also in the way he treats animals.
You also mentioned Moorcock. I've read a lot of Moorcock, but my memories are vague. Could you tell me where he used the trope? From what I remember about the Elric books, for example, gods don't seem to dependant on blieve - they're more general principals with realms of their own who need a contract to "cross over". The theme I remember is more that you shouldn't rely too much on things outside yourself (which includes the gods).
As a counter example, have a quote from TV Tropes, about an anime called
Natsume Yuujinchou:
The second episode of
Natsume Yuujinchou has a very poignant example of this trope (that also doubles as a
Tear Jerker) when a
Youkai that took up residence in a roadside shrine begins to lose his power (and his tether to the living world) as the people who once prayed to him all begin to die of old age. Natsume himself offers to pray to him but the Youkai refuses saying: "It's impossible, because you are my friend."
Here we have a being that depends on believe, but it's also real for non-believers, and it can interact with them. There's an explicit difference between praying at a shrine and being a friend, to the point that the activities are mutually exclusive.
This doesn't strike me as anti-theist. It's world view that doesn't place emphasis on isolated existances. Animism or pantheism in particular don't have a hard time reconciling being dependent on blieve with being real and a person of your own. In shinto-inspired anime, the creation of gods are usually sedimented emotion. You were someone or something
before you were a god, and you don't lose that aspect just because you became one.
A world where everything is connected and everything has divine potential could easily see the god and your imagination of it as two sides of the same coin.
There's also the question of agency. One presumes that a figment of imagination would have no ability to act beyond how its creators would consciously want it to. But it is the direct experience of many writers that characters act as they act rather than following plotlines. Our imaginations are more sophisticated creators than fits the idea of simple order following phantasms.
But whether our imagination is entirely our own is not a given, depending on the world-view in question. Even the word "inspriation" hints at an outside source. Are visions strictly internal, or could they come from outside, etc.
It's possible to draw borderlines between the divine and mundane in different ways, if you draw them in the first place. The evolution of consciousnesses and the evolution of the divine/supernatural might be linked through spiritual laws.
(For me the divine is the figment of someone else's imagination; if I ever met a god, I'd probably not recognise him/her/it as such.)
Flipping this around, there's also the question of what it means that a thing exists. I can't source this unfortunately, but in the writings of a muslim philosopher, I ran into the idea that Allah continually sustains the universe (hence he is called compassionate and merciful). For the universe to end he merely needs to stop expending the effort to keep it going.
This struck me as a very different idea of creation from the, you build it and there it is concept. But it is a creation idea that works for certain kinds of objects.
(That sounds similar to me to how a radical constructivist would think of the world s/he knows.)
On a more relevant note there's the question whether divine imagination has properties that mundane imagination doesn't.
I also wonder how broad or narrow the scope of the words "divine" or "god" are in the context of this thread. I'm reminded of kuwidelu's thread about what makes a god (over in the comparative forum). It's an important question here, since we might be talking past each other.