That's fine. But please do not kid yourself that your mother's visa is totally legal. A bribe has been paid by the agent on your behalf to bypass the standard criteria to obtain the retirement visa. That is a pure simple fact.
I live in Thailand very hassle free and don't have to look over my shoulder. The case I'm familiar with, the guy had been here for eight years on bogus visas before he got caught out. He's now on year 4 of his five year ban! 😆
You're obviously not aware that the 800k must be maintained in the bank account for three months after renewal, and 400k for the next seven months, and then two months before next renewal back to 800k. Can I just ask you what would your mother do if immigration request to see her bank book? My suggestion would be to start packing ready to leave.
And that's good for you. The point I'm making is the process is illegal because it involves paying a government official a bribe. People should be aware of it, and the few cases which do come up results in deportation and 5-year ban. All for the sake of a measly 65k per month.
A requirement of the visa is that the money is held in a bank account in the applicants name. Your mother's visa has been obtained by paying a bribe. This can backfire and she could end up being deported. Probably might never happen, but it's best to advise all the facts
Airport immigration don't give two hoots about 90-dayers or TM30s. Any infringements might come back to bite you later, but these are minor infractions. They're never going to cause any major issues
Don't calculate any more than 12-15 months on an ED visa, which is issued in lots of 90 days, requiring a visit to immigration each time. As Brandon has said expect the third degree and make sure you attend classes and be prepared to be tested on your progress! Your best bet is Elite Visa, and there's rumours that the price will shortly increase due to very high demand for this visa (I think we all know which nationality has caused this!). Although the upfront cost seems high, it's about the daily cost of a decent coffee in your home country