I understand the intention behind purchasing an alternative air ticket out, but that has to be balanced against giving incorrect information to Chinese immigration by (a) the check-in agent entering details into the system before departure and (b) by the passenger on arrival.
On balance I'd rather be giving Chinese immigration the correct information - otherwise it may raise concerns if they're expecting an air-air transit passenger who turns out to be air-sea. Chinese immigration do check and confirm onward travel.
As a back up it might work. If AA refuses uplift the OP could go ahead and buy a fully refundable ticket, be allowed to board, but then present the cruise ticket out of Shanghai once they arrive at the TWOV desk in Shanghai. That may cause some confusion and delays, but better than being denied boarding outright.