I am not sure about the Ramadan rule you mentioned, but for your wife's visa, based on my own experience:
She probably can get the marriage document certified by Saudi Embassy in Beijing through the local notary office in her city/province. You just need to pay some money, around 300RMB I think. The notary office should be able to provide offical receipts for the cost.
But from what I remembered, for your wife to get a Spouse visa, she does not need to get her marriage document certified if you've got yours certified, which I believe had been done already since your iqama is issued. My Saudi visa agent in Beijing just asked the following things from me:
1) The original and two photocopies of your wife's valid passport (valid at least for 6 months and has at least 2 blank visa pages)
2) Two 2" white background certificate photos with your wife's name written on the back in pencil.
3) The electronic copy of the same certificate photo in 2).
4) A copy of the yellowish arabic invitation letter you got from the Saudi Foreign Affairs Department for your wife. (You can scan it here and ask her to print a color copy at her side.)
5) The original and photocopy of her marriage certificate.
6) A photocopy of her Chinese ID card. (front and back)
7) A photocopy of the info page on your passport.(The page with your photo, name, passport expiry date etc.) Again, just scan and print.
8) A photocopy of your Iqama. (front and back)
9) The original of the Intenational Certificate of Vaccination or Prophylaxis (Some people call it "the yellow booklet") and photocopies of its info page(has photo, name etc) and vaccination pages.
10) The original of the Certificate of Health Examination for International Traveller (Some people call it "the red booklet")and all of its pages.
for 9) and 10), she need to find the local entry-exit inspection and quarantine bureau to get them. It took me 1000RMB and 3 hours to do the examination and two extra weekdays to get the red and yellow booklet. She would probably need to prepare two certificate photos and get prepared for a blood sample test and two vaccine injections
Anyways, it's always a good idea to call them for instructions before going.
The visa cost me 330RMB (if you do it through courier deliveries, add another 50RMB for them to send your visa back to you) and it took two weekdays. I actually planned a four-day Beijing trip and got it done within the trip.
Good luck on your application! If you have any other questions, feel free to let me know.