You have to move 16 after she leaves, so you definitely haven't done the 16 villager cycle to get her back.
I'm not in the same boat, but Pecan told me that Whitney is thinking of moving. It drives me nuts that I can't stop her until tomorrow, thanks to Weeding Day.
Awwww I can imagine that it would drive you crazy!
Weeding Day is tomorrow with me (Europe), and today Curlos pinged me he wanted to move out. I'll let him go T_T He's very sweet, but I have not really got a bond with him (plus when he sells me things he always rips me off and I let him XD).
I want to keep a steady flow of new villagers in my town, but there's a few I really want to keep and a few I really wanna get rid of. Curlos falls in neither of these categories, so I'll just... say my happy goodbyes to him. He will be remembered. And I hope a nice new Smug will come to my little town then =D
Back ontopic (I kinda drifted there); The mechanics in ACNL make it so that when your town is full ((9-)10 villagers), villagers will ask to leave, as to make room for new villagers. The game was not originally designed to have the players keep all villagers for an extended period of time, but to let them meet and experience lots of different villagers.
I do believe it is random who wants to leave your town (although with me Fang already asked 5 times, Ankha once, Rory once, Annabelle twice
and saying goodbye she decided to stay urggg and now Curlos), and it's not a sudden move (most of the times).
Accidental move-outs, which also happens to you now, is often because a TT-accident or somehow not talking enough to several or all villagers to find the rumors about a move-out, and then being able to stop it. I believe you can sometimes still stop a villager from moving, as long as he/she is not in boxes yet.