Child Benefits for Foreigners - marriage certificate required?

Hi all, I would really appreciate some advice and info on my situation.

I'm British and living in Thailand with my wife, who is from Myanmar. We have two children born in Thailand. I have been receiving the monthly child benefit payment for our first son for about 2 years with no problem. I recently went to the SSO again for our second son, submitting all the documents. They have called me back and said that, not only am I not entitled for my second son, they will stop the monthly payments for my first son, and I must repay all monthly payments for the past two years (around 17,000 THB). This is because our marriage certificate is not legally recognised in Thailand. (We were married in Thailand, but it was not an "official" wedding - we understood that was not possible for two foreigners).

They have said that we need to get an official marriage document, signed by the recognised official at the local amphur, before the end of the year otherwise will need to return everything. The monthly payments have already stopped.

I have a few questions!...

1. Is it true that the official marriage certificate is required? It seems superfluous when we already have an official Thai birth certificate, with my name on it, for both children, so clearly I am the father. I am also the one making social security payments on a monthly basis. I don't see exactly why the marriage certificate is relevant when we are talking about child benefits. Of course if that is the law then its the law, but I have not been able to find that anywhere.
2. Is it true that I would have to return the money? I have been receiving the monthly benefit for two years for my first son based on the marriage certificate that we submitted, so if this was a mistake, surely it is the mistake of the SSO admin? Can they expect people to be able to pay back two years worth of benefit?
3. A little further from the topic, but does anyone know how to get this legal marriage document for two foreigners? It must be possible somehow if they are requiring it for the child benefit.

Thanks all for the help!