iPad Air 2 used ~2GB of Mobile Data Overnight: what for?
2Gb is in the range of a full software update (or a full backup), and it's unlikely the iPad did that over cellular.

Did you have anything tethered to the iPad? A laptop perhaps? The data limiting doesn't kick in over tethering, and the tethered device thinks it's on wifi. 

If that is the case, you may want to turn off auto-update.