Also, Dumbledore mentioned how he laughed when he found out that Harry was a good version of Riddle, which many readers have taken to refer to this scene in Chapter 17:

Dumbledore started laughing. Laughing a lot harder than Harry would expect, almost howling. It seemed positively undignified. An ancient and powerful wizard ought to chuckle in deep booming tones, not laugh so hard he was gasping for breath. Harry had once literally fallen out of his chair while watching the Marx Brothers movie Duck Soup, and that was how hard Dumbledore was laughing now.

"It's not that funny," Harry said after a while. He was starting to worry about Dumbledore's sanity again.

Dumbledore got himself under control again with a visible effort. "Ah, Harry, one symptom of the disease called wisdom is that you begin laughing at things that no one else thinks is funny, because when you're wise, Harry, you start getting the jokes!" The old wizard wiped tears away from his eyes. "Ah, me. Ah, me. Oft evil will shall evil mar indeed, in very deed."

This was a scene that Quirrell was most definitely not present for, and thus the fact that Mirror!Dumbledore knows about it further suggests that Mirror!Dumbledore was not simply some CEV-illusion of Quirrell's.