getting a new z-visa, location and earliest I can get

Two questions.
1. Which embassy or Consulate should I apply at in America?
I have lived in China for three and a half years, before this, I was a resident of Oregon. While in there, I used my mother's address in Oregon until she returned to our birth state, Montana. Can I still use Oregon, and apply through San Francisco. Do I use Montana, and apply through DC, or would it be some other place... Or does it really matter?

2.
My current z type expires on April 3rd. My current employer will send it in to be canceled the day of, or after. What is the earliest I can apply for a new working type? I don't need confirmation that it has been canceled before I apply for a new one do I? Can I apply before, the day of, or the business day after or must I wait till I receive confirmation? Does it matter?