Only if the other parent is Thai. If so, it’s not a letter from a lawyer you need but rather a special authorisation letter from the district office the child’s house registration booklet falls under, stating travel dates and details and that both parents have given consent. Coming from someone who’s been through the process, best of luck
No, only on Thai embassy facilitated repatriation flights and from some countries there’s even a SQ quota on such flights. From Sydney where I am, the quota is 150 SQ spots out of a capacity of 300