I still think it’s highly suspicious that the Regina/Robin page was never seen until Will arrived from Wonderland. And, more suspicious, that it was found by Robin in the library as Will looked on.
My theory is that when Regina pushed Cora through the mirror she landed in the Hatter’s hallway of doors. The first door she entered was the Author’s room. She found the author writing the Regina/Robin happy ending. Angry at Regina, Cora rips the page from the book, and demands the Author write her a proper royal ending. She’s traps the author in the writer’s room in much the same way she later trapped the Hatter in the hat room.
Cora exits the Author’s room, taking the Regina/Robin page, magics the Author’s door onto a new page, and returns to the Hatter’s hallway of doors.
She then enters Wonderland where she meets Will, our thief, who later steals the Regina/Robin page.
I think our characters’ stories are so intertwined and convoluted because the author hasn’t placed them into their individual books yet. He’s trapped in his own book, Henry’s book. Their stories won’t be complete until the author places each story into the character’s individual storybook. It would be a bittersweet ending to ONCE if our characters said goodbye to each other, as they individually chose to say goodbye to Storybrooke and entered their individual storybook where they could live happily ever after.