Her behavior is a defense mechanism (when she is not wanting to deal with adult issues that she is finding stressful and anxiety provoking) Regressing to that level can also be a way of diverting the attention from the topic and placing it onto herself.
Perhaps she is not ready (really interested in having a child or feels overwhelmed by the idea especially if she herself feels that she was "robbed of her childhood." Taking care of a child will completely wipe out all fantasies of making up for her lost childhood. Of course, even though irrational, to her these fantasies are real. She may believe that there are things that she always wanted to do that she may not once there is a child to take care of. Also, she may feel that she will have to share your attention with the child.
She does know when it is appropriate not to use that baby voice/talking and she is most likely using it for a purpose since behavior is purposeful (even when it seems unhealthy or uncalled for)
Since you've shared that she had been abused, hopefully she had gotten some help for this (if not, it is never late. After all, she would certainly want some normalcy in her life at this point)
Perhaps you would want to try to ignore her behavior when it starts (instead of being rational with her or fighting) You have already told her how you feel and she is ignoring it. Let her know that you will talk to her when she is ready and see if she responds to that approach. You will be trying to extinguish the behavior by not feeding into it. When you engage her in it by trying to reason with her, that in itself changes the topic and she still wins.