What Are the Rules of Something from Nothing?

[Esquilax]

And if anything can always have existed, it might just as well be the universe itself

The reason why the universe could not always have existed because if it did you would have had an eternity to come into being before now, so you should have already happened.

You don't understand how time works, do you?

An eternity is enough time for everything to have happened, yes, but that doesn't mean it doesn't happen along a linear timeline, nor does it tell you how much of that eternity has elapsed. Your comment makes no sense.