Have you tried
www.travelzen.com using Firefox? They are competitors to ctrip though not as well known. Sometimes I've found better deals with them, sometimes not. They will take foreign credit cards for online bookings. I honestly can't recall whether I used Firefox or IE to do my bookings (I use both), but might be worth a shot. Or call and book by phone.
Finally, have you tried the old-fashioned method of calling China Southern on the phone to do the booking? This may not work so well if you can't get a decent English speaker, and also if the special prices are not honored for direct phone bookings. Maybe also worth a shot though--not sure if your previous call to CZ was to general Customer Service or to reservation line.