Stay in China for 90 days? What if I get a job?

Hey everyone, I am hoping for help with one or both of my questions. First, can I stay in China for 90 days? I recently got my visa back and it says one 90 day entry. I have a hotel for 27 days, but can I move my flight date and stay for the full 90? I ask because I thought you could only stay 30 days total. I am a U.S. citizen, if thateans anything. Also, if I get a job while I am in China, do I have to come back to the U.S. and apply for a work Z? Or can I get that switched right over in Shanghai? Thank you for any and all information!!