#4 Alexa Bliss will be using a different move set, in the thrall of The Fiend
There are so many signature moves of The Fiend that Alexa Bliss can adopt, as her descent into the darkness takes her to places that she has never been. Yes, the Mandible Claw immediately comes to mind, but she could also convulse by the turnbuckle in the manner that Bray Wyatt does, or even execute the running senton similar to Bray Wyatt's offensive move, although she may not necessarily have the bulk to make it happen.
No Alexa Bliss may not be the physical manifestation of Sister Abigail, because it has been mentioned several times now that Sister Abigail does not exist, but she could be possessed by the demonic spirit of Sister Abigail or indeed, Abby The Witch, in storyline. What will be awkward to see is how Alexa Bliss manages to balance this new character with her brand new podcast which may not necessarily be the smoothest transition, based on just how different the two WWE personas will be.