If you are granted a fiance visa, marry in the US and then return to live in Thailand how can that Thai national return to The US?

Obtain a normal tourist visa. Proof of her intent to remain/return to Thailand will be important and your ties here/outside US will help. Also the fact she had obtained clearance to US and departed paperwork should greatly help. For those who marry overseas it is more difficult to prove but she already had the opportunity to remain in the US and departed so it should not be as hard to obtain a tourist visa. The concern for tourist visa issue is the normal "will they return" and "are they using this to bypass the normal immigrant visa procedures". Your spouse would have positive points on both.