I can say first-hand that I've been sent back when I decided to play dumb and headed to the shortest line, which may have been for refunds I forget.

Are you certain that as a foreigner with a just confirmation code "in some cities around Shanghai, you do not even need a paper ticket anymore." At a minumum I believe you have to go through the photo session on the 12306 app. Not likely the OP is going to do this.