If she's 16.7 lbs now, I'd say there's no chance she'll be small enough to fit in-cabin in December. Even now she barely qualifies as small enough. I'd say that if you really want to take her, plan to ship her in the hold... for her sake and yours.

It's not about leniency, it's about her being able to actually fit in a bag under the seat. If she can't physically fit, it'll be miserable for her, even if the airline allows it.

Welcome to FT!