Good points you bring up, Barbara. I was just thinking about how Marianne seemed to treat Norland itself as her first love. (Elinor teases her about her appreciation of the dead leaves there, etc.) And while I don't know why she wouldn't just put a picture of the estate in a locket, the thought occurred to me she might associate her affection for Norland and for her great-uncle together.

Some LOL lines (for me) that Elinor said occur right here:
[Ch12: speaking to Margaret about her assertion Willoughby and Marianne are engaged] "You have said so," replied Elinor, "almost every day since they first met on High-church Down; and they had not known each other a week, I believe, before you were certain that Marianne wore his picture round her neck; but it turned out to be only the miniature of our great uncle."
Just the idea of romantic love associated with a great uncle was humorous... and Elinor's remark regarding Willougby cutting a lock of Marianne's hair:

"Take care, Margaret. It may be only the hair of some great uncle of his."