Did you check the mirror for excessive dirt and/or damage? Cleaning the screen is easy, corners and edges of the mirror not so much, so maybe the previous owner did a quick clean that didn't include the whole mirror.
edit: Oh, and that F3 doesn't look so worn out.
That mirror would have to be utterly encrusted to block that much light. Dust and smudges? Doubtful. Still wondering about mirror alignment issues and whether the viewfinder image is squared up.