Anonymous participant 833 For example to avoid the withholding tax for bank interest (up to 20k THB per year). Or for identification purposes for foreign financial accounts. If you are a tax resident of Thailand you can get a tax id but being a tax resident doesn't mean that you have to file a tax return necessarily.
Anonymous participant 833 You don't need to do a tax return just because you have a tax id number. If you don't have any taxable income there is no need to file.
Theoretically he can do it but the problem will be opening the required bank account on a tourist visa. It's become very difficult to do that recently.
Technically the onward ticket is only required for entry by visa exemption. It shouldn't be required if you hold any type of visa. However I have heard stories of visa holders being asked for it anyway.
Were you really planning to leave the country within 15 days? I can understand the officer's confusion after you said that. There is no reason for the extention in that case.
It doesn't really. I read somewhere to do that the first time and just kept doing it. Probably better to use a driver's licence or utility bill although neither of those really proves your current location either. As I remember there is no official guidance on what to submit.
When applying for TR and ED visas in the past I just uploaded my passport main page as proof of current location. Canadian passport and was in Canada applying through evisa website. Probably not the correct way but it was approved every time (3 times).