I don't know whether you can apply I824 before your I485 is approved. I don't know how long it will take in Guangzhou. There rarely any example in the Internet.
I would suggest you find a good lawyer who has experience on Following To Join case.
Please share your progress if possible, thanks.
The botten line is that your hu*****and doesn't wait for a long time which happens if the marriage after one of the spouse get Green Card.