Short answer: no, an Amity company can't hold land. The treaty gives Americans national treatment for running businesses, but it expressly leaves out the exploitation of land (Article IV). The Land Code treats a company with more than 49% foreign capital as a foreigner [Land Code s.97]. A Thai subsidiary owned by that company is caught too [s.98].
It also isn't a "gray area." Land held through a Thai company on a foreigner's behalf can be ordered sold, and acting as the nominee is a criminal offence [Land Code s.96, s.113].
One correction to the thread: a lease is capped at 30 years [Civil and Commercial Code s.540]. A "30+30+30" or "99-year" lease is 30 years plus renewal promises, and those may not bind a later buyer of the land. The lawful routes are a registered lease, a superficies (you own the house), a usufruct, or a Sap-Ing-Sith. Details here: https://proviso-consulting.net/insights/can-foreigners-own-land-in-thailand