If a Thai national (with valid Thai passport) were to enter Thailand visa exempt with a US passport would they have to leave and re-enter with their Thai passport within 30 days or can this situation be straightened out within Thailand?
TLDR : Answer Summary
A Thai national entering Thailand on a US passport can address their visa status without leaving the country. They are eligible for a one-year extension of stay as a Thai citizen at the local immigration office, which costs 1,900 THB, provided they can prove their Thai nationality.