How is it an illegal purchase? True foreigners aren't allowed to buy land. But a corporation is a legal person distinct from its owners (something you clearly don't comprehend based on what you are saying), and it's the one owning the land, which it can because a Thai owns 51% of the shares of the corporation. It's also totally legal to be a minority foreign shareholder with voting rights while the majority shareholder doesn't have voting rights unless Thailand specifically amends the law to close this loophole. It's done all the time in many jurisdictions.
Also don't forget to hold regular shareholder meetings, board of director meetings, and pass resolutions. Because that's what a real company does as opposed to a shell company which is somebody's alter ego and thus has no legal personality of its own.
If it's done by a reputable lawyer, then it's just as legal as tax avoidance strategies US corporations use to minimize their tax payments. They're also loopholes, but perfectly legal. The government is of course free to close them. Then you'd have to come up with an alternative strategy, such as give the Thai shareholder voting rights too. You'd really have to make sure to use someone trustworthy in such a circumstance, but it's still a better option than using a spouse due to the arm's length nature of a properly managed corporation.
In the end though, you're right that a court in Thailand could easily set aside the entire structure because there is no respect for the rule of law in Thailand like there would be in a place like HK. Everyone in HK is equal before the court regardless of race or nationality and the judge interprets the law based on previous case law, but I get the feeling it doesn't work that way in Thailand.
