oh course the school told you it's legal. They are the ones selling it...lololol. Just understand that when scams like this are shut down by the police and immigration those students involved are deported.
if you have your exit flight out of Thailand booked with the same airline you fly in with you will have no problems with a flight out more than 60 days later if it is with the same airline.
You do have to keep the bt500,000+ in the bank. You can't withdraw it all once you get the visa. It is not publicized how they will enforce this yet, but they will.
You seem to think like this is a third world country. The BOI does not allow any investment they do not approve of to begin with. Secondly there is a background investigation that takes about three months. There is no restriction on who can get one, but they are aimed at corporate CEOs, and directors of major international business in Thailand, and they do not want them to lose money on these allowed investments. It's actually the only visa you can make money on, without actually working. From my experience I would expect someone with your attitude about the safety of 10M baht to think cryptocurrency is a good investment. Thailand isn't Cambodia, Laos, or Myanmar, where you can expect to get ripped off. Thailand deals with too many countries commercially to do that kind of cr*p anymore.
the brand new DTV visa ended that, if they qualify. Before July 15th they technically were illegal, staying here mostly as a tourist, or on an ED visa, yet working, and using either simply to stay here long term, as technically any work requires the correct visa, and tourist entries, or an ED visa do not allow working of any kind technically.
you are out of date, and in fact the only limit was before July 15 when only two land border crossings were allowed in a calendar year. That no longer exists. There never has been a limit on entries by air, or sea. Where ever you read that is full of BS. The big reason it was changed is the fact that many people visit more than two countries that border Thailand while they base out of Thailand. The only unwritten rule now is if you are trying to stay long term in Thailand on tourist entries you will at some point be refused entry until you get the correct visa. That detail has not changed. Immigration is well versed on this, and every angle used to stay long term in Thailand as a tourist, without really following a pattern a real tourist follows.