For me, resetting my browser was the key to the missing admin bar.
I had been logged in before I moved the site to a different directory, then made the directory change when the website went live that afternoon. The admin bar disappeared. When I read that yours had suddenly shown back up after about a week, I expected it was due to cookies expiring. I reset Safari, and I am able to see the admin bar again.
Not sure about the theme preview issue, however. I haven’t run into that.
