It might be advisable for you to actually visit a decent travel agent (Trailfinders always springs to mind, I'm assuming you're in the UK). They can quickly find you the best fare and option given your requirements - mileage-earning in Flying Blue and ability to select seats as part of the booking.

Print off the China Southern FB earning tables and bring them along - any fare classes not listed in the table will not earn, and read carefully for any other gotchas/exceptions/exclusions

http://partnersearch.ehn.klm.com/uk_...l=5&p=1&eb=all

http://www.flyingblue.com/earn-miles...-airlines.html

you seem to already know that any flight that does not earn miles will not count as a level segment and will therefore not help your husband to Gold status.