Outbound ticket needed?

Hello!
I already have been issued an S visa and am traveling to China next week. I have two questions:
1) Will I need to present proof of onward travel (i.e., a roundtrip plane ticket) when I land in China in order to be allowed in?
2) If I choose to fly into Shanghai from one country and out to another within 144 hours, will the immigration official issue me a transit visa? Or will they still use and thus invalidate the S I already have in my passport?
Thank you!