Clara according to what has been said will never go back.

Moffat says a lot of things! 🙂

 This article  has Moffat doing his typical: “In fact, we may not have seen the last of Clara, with Moffat refusing to entirely rule out a comeback for the character.“It can never be true in Doctor Who – you can never say that someone can never come back. They always can, because it’s a show about time travel. So it’s entirely possible." 

So I take the never coming back with a grain of salt. Do I think she’s leaving, yup! I bet she comes back for the Doctor’s regeneration a la Amy. And I don’t think she’s dying as in dead dead. Just my opinion.

Your other ask about Jenna in a waitress, could be anything. One of her echoes? A dream? A memory wiped Clara? Or even Clara herself? It’s the guessing that is fun 🙂