I found proof that Clara’s TARDIS is the Diner from The Impossible Astronaut.

I think that we were here, you know? I think that we were here together once. I’m sure I’ll remember. Over here.
(The tables by the wall.)
DOCTOR: Stupid Doctor. Amy and Rory. It was Amy and Rory.
CLARA: What about your Tardis, hey? Have you found that yet?
DOCTOR: No. Somebody’s moved it from London. I’m still looking. But this diner. It wasn’t always here, was it? It used to be on the other side of the hill.
CLARA: Well, maybe someone will find your Tardis for you.

Reports are, this next season will both be Capaldi’s and Moffat’s last series. Capaldi is recovering from on-set injuries. And Capaldi and Coleman have openly said that the end of the series was written so Clara can come back.

I think the finale episodes for the 12th will be amazing. I think most, if not all, or the Moffat era companions and major allies will be back. We still have one mystery, regarding a connection between River and Clara. Remember this from The Name of the Doctor?

RIVER: Oh, there’s one more thing.
DOCTOR: Isn’t there always?
RIVER: I was mentally linked with Clara. If she’s really dead, then how can I still be here?
DOCTOR: Okay, how?
RIVER: Spoilers. Goodbye, sweetie.

I think there is some connection here, that will link Clara and Ashildr to the Ponds. I feel confident, from the script, that Clara and Ashildr were there in the Impossible Astronaut.

As to why the restaurant had other employees, well, it could be as simple as Clara and Ashildr decided to make some cash using the TARDIS.