Look at this from your DD's perspective: Talking is hard. Why talk when there's signing?
The primary motivation for kids to work on their linguistics is the need to be understood. She doesn't have that need, and so, she's not working on it. It's not a priority.
One way you might try to make it a priority for her again is to pretend you forgot sign language.
Otherwise, the only thing that seemed like any kind of real "red flag" is her fear of strangers. Apart from that, it sounds like she's doing awesome.
