LAST EDITED ON 12-16-14 AT 11:16 AM (EST)I think if Missy does volunteer it will be because the pain of her injury gets worse and she feels that immediate medical attention is in order.
The thing is, we know from Jon that one of them (i.e. Baylor) volunteering to leave before F3 was part of their plan. With Missy's injury she might decide maybe it's just as well to flip the script.
Otherwise Keith would probably get booted at F5. Which is certainly possible. It wouldn't seem a blindside to the general viewership. ETA: Except we now know that Keith gets to F4, so forget that idea.