In America it takes over 10 years for medicine to be approved. You're moving to a different country with the different medical system and different treatments as well as medications. Where it's true you probably can get those medicines imported there at a higher cost, they might have better local medicines or newer treatments than in America. When you arrive you might have her rechecked and see if they offer a better treatment here
Correct the immigration officer has the final say. If for some reason they think you're undesirable yes they cannot allow you in the country. However the new rules which were changed under Thai law allows unlimited entrances on land border with the new 60 day fees on arrival