The plain facts are we can't entertain thoughts of how big the universe is. We just cannot comprehend it. Our finite minds simply do not have the capacity to think that big.
If "we" refers to you and me (at least), that's not plain fact at all, as I am entertaining thoughts of how big the universe is right now. By simply adding a few zeros to the end of various numbers involved, I can think of far bigger things as well.
It's beyond our reasoning powers. We cannot imagine infinity. It leaves no room for expansion.
You can't logically conclude "It leaves no room for expansion" without reasoning about infinity. Are there more than 10 grains of sand on earth? Yes. More than 10 thousand? Yes. More than 10 million? Yes. After a few more steps, the correct number is reached (give or take a factor of 1000). This demonstrates it's easy to think of quite large finite values. Now consider all the positions available for grains of sand on earth. That's easy, but there are infinitely many such positions, and still room for expansion. Moreover, there are numerous books that contain reasoning about such infinities. Their existence proves that reasoning about infinity can be done by humans. You can read them and do such reasoning too.
Why not accept that there may be an entity far more intelligent than ourselves who may indeed have created us?
Why not accept that the earth supports a large population of invisible unicorns or any other absurdity one can invent? Would you prefer visible unicorns? There are several references to unicorns in the Bible.
In fact logic tells us that an object only 8000 miles in diameter cannot remain molten at the centre (which the earth is) for millions of years.
Logic doesn't tell us that, and available evidence indicates the earth's centre is solid anyway.
I have discovered that he has never let me down.
That's logically compatible with his non-existence. Also, I would classify "letting you down" as including allowing you to declare as "fact" or "plain fact" various things that you shortly afterwards admit are mistakes.
Evolution also presupposes that things are getting better.
No, it doesn't, so your reasoning is fallacious.
It is all a mystery to me. Nothing is cut and dried, or put into pat theories.
Both "It is all a mystery to me" and "Nothing is cut and dried" are pat theories of yours, so your reasoning is again fallacious (even if those theories are correct, which is arguably logically impossible).
It's all beyond any of us for any of us to know definitely about these things.
And you definitely know that, do you? How come it's not beyond you to do so?