Dawson is definitely playing someone from Daredevil though, so it's more likely that she's Elektra, since they began casting for Jessica Jones after Dawson was cast in her Daredevil role. If Cage has already been cast, it's reasonable to think Jones and Iron Fist are cast as well, since Cage is being released last.
Also, the specific wording of Elba's statement actually means it still might be him. He said that an actor had already signed on. The phrasing of that is vague enough that the person in question could still be him. He didn't say "someone else signed on", he said an actor had already signed. He didn't actually answer whether he was cast...interesting.
I'm predicting a big on-stage roll call of the Defenders a 'la to the Avengers panel from SDCC10, announcing the new actors at the end of the Daredevil panel, with early logos and a first photo of Daredevil in his show.
I'm also willing to bet that they'll announce the Captain America 3 title and confirm Phoenix as Doctor Strange, and maybe (big maybe) announce the other 2017 films outside of Guardians 2, which I'm betting will be Thor 3 and Inhumans/Black Panther/Captain Marvel.