It's absolutely free. Make sure you are on the actually government site and not a third party scan site. The TDAC is specifically for Thailand and has nothing to do with Korea.
It is trivial to fill out, under 5 minutes if you have your flight info and initial address.
Look on YouTube, I'm sure someone made a video walkthrough
No, you need to start with the 800k. The 65k/mo is only valid after you have been here a year and want to extend again. At least that is how the bank explained it to me.
Each visa and extension is individual. You each must meet the requirements individually. Your marriage ( assuming neither of you is Thai ) is not considered