Everything is checked every time. They are all BRAND NEW applications.
First you apply for the visa from the embassy, and meet whatever requirements they have. Each embassy sets their own requirements so you need to see what your embassy wants.
Then when you arrive in Thailand, you will be applying for the extension from immigration. This is a brand new application, and will require everything that your immigration office requires. Each immigration office sets their own requirements so you should go to your immigration office ahead of time and ask them for the handout of requirements for a marriage extension. If using 400,000 baht, the money will need to be in a Thai bank account in your name only at this point, and must have been in the account for 2 months already at the time you apply.
Use whichever is easiest for you to get that meets the BOI requirements. You're not required to use that insurance if something happens, it's just a visa requirement.