What were the main ingredients of the Thai cuisine before global trade?
Thailand, if you define it as the country where Thai is the dominant language, is a relatively new nation. Sukhothai was established in 1238 and from the beginning there was international trade. As soon as ingredients from the new world started to spread to the old world they would have arrived in "Siam/Thailand" and all other parts of Southeast Asia. It doesn't take much to spread new plant ingredients, just a few seeds and it would be quickly cultivated in mass and dispersed everywhere. It doesn't even require ongoing trade.

Of course the land was always there before the Thai arrived so if by "Thai nation" you are also talking about the Khmer, Mon, Malays and other groups that lived in and controlled the territory of modern-day Thailand, then your question does not pertain stricly to Thailand or "Thai cuisine."

Even if you look only at the Thai-speaking peoples, what do you consider "traditional Thai cuisine?" Have you considered that there is not one Thai group but multiple? Chiang Mai and northern Thailand was its own kingdom and has its own cuisine. Northeast Thailand was part of the Lao kingdoms. There are also some Shan and other tribal upland Thai groups. All are "Thai" and all have their own cusine.