I always got the impression she does talk to Jada. I feel like she's selective about who she's hiding her thoughts from.
It's not clear if she talks to Jada, but IMO it's been fairly strongly implied that she does; when the MC asks her if Ellie talks to her, she refuses to answer. If Ellie
didn't, I'd think she'd just say no.
It's also possible she actually talks to Kath sometimes; that cryptic statement she made about Ellie being
perfectly normal makes me suspicious.
I'm also in the camp that thinks she probably will talk before the story is over, and also that she'll probably keep a lot of her non-verbal mannerisms she picked up during her quiet period.