Re: Lucy's purpose
I'm more inclined to believe the "total airhead" approach. She didnt even remember Mystogan made her take the Exballs before sending her to Edoras. But the "Came to terms with mothers death" and "Too traumatic to make a connection" approaches do hold some value, too. Maybe it is a combination of two or more reasons?
Regardless, you do make an interesting point on why she didn't react when Wendy mentioned that date. And before that, apparently Natsu mentioned that Gazille's Dragon had disappeared the same day, and she didn't even remember.
Or, y'know, the most probable option, Mashima hasn't decided what the hell to do yet. But for the Dragon's plot that doesn't seem to be the case (Or at least, not as much as for other stories). He seems to have a general idea of what he's doing there. Not that it means he's got the details figured out, but still.