Each embassy differs in the seasoning of the 500k THB. Asian embassies often require 3-6 months. Many US embassies are ok with a final balance of the required amount.
Business registration, permission to work remotely in Thailand, detail the online business activity and that you will not engage in business activity whilst in Thailand. I applied in Taiwan and there are many posts in the threads that will help you.
Yeah, they love their paperwork. It is not the best for SP because of the demands you mentioned. I have helped friends from Japan get their visa from Taiwan since going online and recently as last month, so I know what it takes to get approved, it is not that different to when I got it except for the immigration agency part and longer wait for interview. You were unlucky with your tough interview