your marriage cert certainly wasn’t stamped by City Hall. Are you sure you didn’t have to get a Khor Ror 22? Certificate of family status confirming your still legally married. This is issued at City Hall and at Banglamung civic office and has been required for marriage extensions for many years.
Entirely depends on the Embassy you apply through. The period required for class enrollment is Embassy specific. Once the visa is granted there is no additional requirements to fulfill.