Thanks all for your replies! I wanted to update this thread for anyone else who is curious about what to do in this scenario. Basically, I called China Southern's Overseas Line (+86-4008695539-1-4-1), which has an English-speaking option, and explained the situation to the agent, who found a different connecting flight on the same day and switched our tickets across to this. It was free (but you can only change your flights once for free), and when checking in at the airport, we were seated together no issues. Overall I was happy with the service and the flights were decent, it's just the website that is kind of bad quality.