"I know someone who" and how many years back was that? You cannot enter unlimited with visa exempts any more. You will be denied entry after the third time
that's not how to proceed. If you got two citizenships, you enter and leave Thailand with the Thai passport, book the flights and check in to the flights with the UK passport. It's the common practise since years
it actually nothing new, Immigration officers used to give their ok when one showed both passports. The good old way I described still works, according to these "not so news". You enter Thailand with the Thai passport and leave Thailand with the Thai passport. Why friggle around? There is no "discrimination" by this method
if you ENTER Thailand on your UK passport, you will need to LEAVE it on your UK passport. . . . . . . . . . only on your NEXT trip, you can check-in with the Thai passport, leave the UK on your UK passport, enter Thailand with the Thai passport. After your holiday, check-in on your UK passport, leave Thailand on your Thai passport and enter UK on your UK passport
yes, you could be right, normally when you hold a valid visa, the airline doesn't ask for an onward travel proof. However I guess the ticket "within 60 days" is needed for the METV visa application, which is a whole different story
there is no guarantee an airline will accept a bus ticket for an onward travel proof, because they could say that the bus will only get you to the border but not across it 😃 The only safe and 100% guaranteed onward travel proof is a "real" cheap one way ticket out of Thailand. You can use Air Asia from Hat Yai to Kuala Lumpur as a perfect proof. Why do we hear about "onwardtickets" soon to get refused? Because they are not "real" flights but just reservations, one can clearly sort them out in the TIMATIC system, and since the beginning of the year the fine for violating IATA rules has risen five-fold, it can be up to 10.000.- USD per case.
get a Thai passport for the daugther. (she automatically is a Thai citizen if she has a Thai mother). Then exit Thailand on your foreign passport, making your stay permits invalid (hopefully you haven't bought any re-entry permits, because an exit under an existing re-entry permit would not invalidate the stay permit and you would have to ride it out until the "admitted until" date) Okay so exit, and both of you re-enter on your Thai passport. Problem solved