That it normally would make more sense to create a new identity. But RJ's and BS' comments have pretty much excluded this possibility and point to Mesaana being an AS.

That we haven't seen Danelle or heard of her since ACoS, prologue, while we see Norina at least in two scenes in KoD again, might - from a writer's PoV - rather indicate that Mesaana is Norina. Most readers aren't that involved with all the mysteries & characters in the series, as we are. It would, so to speak, "only be fair" to show Mesaana's alter ego in recent book to allow even such casual readers to figure it out or at least remember who Mesaana's alter ego was.