So, yes, in summary this is what has been learned:

1. booking on some other portal or provider lets you book tickets with only name (like Lastminute.com)
2. without the passport No, Expiry date, date of birth and name Shenzhen Airlines website is useless, and will just give errors
3. Calling +86 10 95361 was a way to get those details added.
4. That however doesn work still in my case (maybe because travelling with a minor!?)
5. An agent (press 2 for English) can book seats and special meals, but only seats that are free.
6. Only the seats are confirmed, no confirmation of special meals
7. Air China's website seems to go a step further and lets you enter the seat selection process, but now says there is already a seat reservation (which should be cancelled before making a new seat reservation
8. There is no optiont o cancel the seat reservation as far as I can see, even though my booking does show
9. emails go unanswered ([email protected] | no reply; [email protected] | no reply; [email protected] | mailbox over quota; [email protected] | no reply)
10. the Air China account seems to link in with the Shenzhen Air account, but still no use.