I understand. Just going on what my employer's HR was told my Immigration at Chaengwattana when we queried whether we needed to renew his Extension. Maybe they just wanted the 1,900 baht!
I guess you're not extending at the main Immigration office in Bangkok, as they have extended my 3 year old's Extension twice now. Or maybe extending a visa is different. Either way, it was recommended to me that I should keep his immigration status legal, even though there is no fine or ban, as he would still get stamped as overstaying in his passport.
Chaengwattana accepted our UK marriage certificate in English, although it had to be legalised and the legalisation had to be certified by the Thai Embassy in London.