The Immaculate Conception a "perfectly ludicrous proposition", Steve? I'd have thought you'd actually agree with that one. It declares that the mother of Jesus was completely free of "original sin", and since I rather suspect you might disagree with the notion of "original sin" in the first place, you'd surely have no reason to take issue with that.
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As for the suggestion that very few people have come up with the idea of God de novo, you are likely right. But then, if that idea is already all around them, how could they?

However the suggestion that if you leave the idea of God out of it, things fall into a neat comprehensible pattern doesn't really stand up too well.

Here's a chunk from an article about Cosmology. (Which incidentally shares my suggestiin that cosmology can be seen as a branch of Metaphysics) It is interesting to click on that, if only to see its summary of an imposing list of som 44 Cosmologies that have floated around over the centuries, most of them put forward by scientists in the last century. I very much doubt if many people will have come up with any of those on their own either.

Cosmology deals with the world as the totality of space, time and all phenomena. Historically, it has had quite a broad scope, and in many cases was founded in religion.[citation needed] The ancient Greeks did not draw a distinction between this use and their model for the cosmos.[citation needed] However, in modern use metaphysical cosmology addresses questions about the Universe which are beyond the scope of science. It is distinguished from religious cosmology in that it approaches these questions using philosophical methods like dialectics. Modern metaphysical cosmology tries to address questions such as:

What is the origin of the Universe? What is its first cause? Is its existence necessary? (see monism, pantheism, emanationism and creationism)
What are the ultimate material components of the Universe? (see mechanism, dynamism, hylomorphism, atomism)
What is the ultimate reason for the existence of the Universe? Does the cosmos have a purpose? (see teleology)
Does the existence of consciousness have a purpose? How do we know what we know about the totality of the cosmos? Does cosmological reasoning reveal metaphysical truths? (see epistemology)