Were the present reign to end without the King's nominating a successor I suspect that Princess Sirindhorn would either be appointed Regent for the King's youngest son or as Queen, herself (it depends on the strengths of various Army factions at the time). In the latter case I think she would bring home the King's sons in exile to see if one of them were suitable for eventual succession. (There are rumours of widespread concern about the health of the King's youngest son.)

The possibility that the King's oldest daughter might succeed also cannot be discounted, as you say.