I don't have much time to respond so this will be short...
First, she trusts you, at least on some level. She created an alter just for you. Don't expect much from the new alter, she's probably there for the short term.
Second, she probably has a construct in her head, like a castle or a cave or a big house where all the alters live. If she can give you a clue of where she goes when she's not "out" then you'll have something that's a little more tangible (in her world) to begin helping her realize she's not alone.
Lastly, DID is a protective mechanism; it allowed the child to survive. Unfortunately, people who have suffered childhood abuse often have poor barriers and they often perceive the good guys as bad guys and the bad guys as good guys. Be very wary of the EA partner. Google "grooming" in the context of childhood sexual abuse.
