We came in visa exempt 30 days and were asked to show onward ticket (within 30 days if arrival)when checking in for flight with Singapore airlines. Your flight out does not have to be your home country. Also note on arrival they can ask to show you have equivalent of 20,000 baht in $$ per person.
The Covid extension first time is from application date and yes date applied counts as a day nothing to do with test N Go and test days, if 2nd Covid extension then it's added to existing date. Your passport would have been stamped on arrival then again when you did 30 day extension