Three wasn’t meant to go into the temple. The Hybrid said the secrets would be revealed to the chosen one (Baltar.) Head-six confirmed this. The Hybrid sent Baltar and the Cylons to the temple, and she takes her orders from the God. As Baltar says, “it’s not chance” that they all meet there at that exact time.

He ain’t kidding.

That Opera House is also seen by Baltar in the visions given to him by his inner Six, with Hera in a crib. No, it’s no fake out. Three didn’t make it up. And don’t forget that Tyrol, member of the Final Five, had programming to make him find the temple, and programming that made him disobey direct orders to blow it up. No, the temple “of five” — no coincidence that — is definitely tied to the Final Five, and it’s 4,000 years old, and it’s built by people from Earth, as #3 says.

And the Final Five made themselves trigger at the Ionian Nebula, a place they would only go after seeing the nova at the Temple of Five. For this to be a fake out would be a serious dramatic flaw.

As for the numbering, it seems to be pretty accepted they assigned the numbers at random, and only later decided to make 5 of them special, but it was too late, they already had a 3, 6 and 8 — no way to make the 5 be contiguous.

I never heard of a backstory of collapse, but it does seem likely, since their records of the Kobol days are scrolls and legends, not computer files. But oddly they had a virus from those days in the computer files, an Earth virus to boot.