Yes, and the country where the capital gain is realised is the country where the tax is paid. The OP is on a retirement visa. Foreigners are only assessed on remittances sent to Thailand. Anything else which remains in another country is beyond the reach of Thailand. Honestly, some people have no idea
Yes, in the country where the asset is held. What does that have to do with Thailand. I sold an apartment in Australia last year and made a profit of AYD$500,000. That's assessable as Capital Gains Tax in Australia. What the f*ck does that have to do with Thailand?
It's not unclear at all. If money you bring into Thailand has already been taxed, under your country's DTA it cannot be taxed twice. However, if you haven't paid tax, it becomes assessable income in Thailand and will probably be taxed here
Your overseas income has nothing to do with Thai taxation. You're another one who's listening to the bs being spread by tax agents touting for business