Thais can few foreigners as scary. Bigger than Thais and if angry looking Thais think I won't talk to them. Thai women have told me a lot of foreign guys look scary.
Easy for staff to ask at check in. Thai websites only mention international flights. I doubt you need anything domestic but maybe a phone call. Staff is very relaxed on domestic flights.
A Thai person who flies would know domestic flying regulations. Maybe buses are the same.
Nothing to do with farang. It's a non parent travelling with a child.
How so? They have been living there for 30 or 40 years. Should know. International flights require letter and phone number to confirm. Domestic maybe nothing but a Thai who flies would know.