Funny that you wrote that I am wrong but said mostly same that I said, but also wrote wrong. Show me stamp for 90 days stay on TR visa... Or on DTV.
Once again - Thai visa has validity date, and this is date when you can enter. And based on visa type you get stay permit stamp. It works not as Schengen visa where date is allowed stay period also. What wrong you see here?
ask embassy. I know cases where it was additional 15k for each adult only, and cases where 15k for each dependent, and hear that some can use 15k for main holder for all