Can you visit the stadium when there is no tennis on??

I think so yes

You have schedule a tour for stadium access. I know this b/c my professor was a director their

Surrounding Yes.. stadium No

I have been to the usta site many times when the us open is not on, and the stadium looks shut to me, so I doubt it...

Yes everybody can

Well, I believe you can visit the surrounding area and the USTA grounds freely, but I don't think you can go into the stadium unless there is a game or have some kind of special access.