Though I agree Daniel with what you wrote about redirects, we do have a policy (I believe I have read this somewhere) that if you create a redirect in the case of two items about the same thing, then it should always be the older item number that survives (and the merge gadget is supposed to check for this). Maybe that's too much detail information though. So for the given example "Moon" was created first and thus it is "Moon" id that should survive, whether or not the label is set to "The Moon" later.
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Yes, page IDs survive renaming/moving the page.
> 2. What if we have page "Moon" with ID=1. Someone creates a second-page
> "The_Moon" with ID=2. Is it possible that page "Moon" is transformed into a
> redirect? Then, "Moon" would be redirecting to page "The_Moon"?
Yes, pages can become redirects.
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