Yes, money earned in 2025 and brought into Thailand in 2027 is taxable. If you have to pay tax depends on the situation and DTA. Why do you think it should not be taxable? I t is up to you to prove you already had this money before 2024.
The only "rule" wirh 3 months is that applications with an INTENDED date of arrival are cancelled with no refund by most embassies. Intentions can change with or without reasons. The only thing I can think about is that in case of soft power you might get questions why you do not following the training, but even that is highly unlikely.
That just depends on the situation, some people prefer to pay lower Thai taxes and not high taxes in their home country, and depending on DTA and type of income there are possibilities
You may be right that Thailand does not track the money, but you are responsible to tell them, and tax fraud might be punished, all up to you if you want to take that risk.
money brought in by creditcard is also bringing in money. It does not need to be cash. And living in Thailand, not working in Thailand, and not bringing in money from outside looks quite suspicious, don't you think?