Can my husband extend his Non-O visa or obtain a 30-day extension while we await our marriage certificate?

If your husband is indeed on a Non-OA Visa with a 21-Sep expiry date and not an Extension of Stay stamp that ends on 21-Sep getting new 12 month insurance and doing a border bounce before the Visa expires gets a new 12 month entry stamp.

You can’t get a 30 day extension off a Non O visa. Probably the best option would be for your husband to border bounce and come in on a new exempt entry stamp for 60 days. If needed that can be extended for 30 days.

You could then apply for an in country Non O if your marriage certificate arrives in time. If it turns out that you have the certificate, but not enough time left to convert then you can get a 60 day visit Thai family extension.