You have been granted a Tourist Visa. It looks like they will not process your DTV until it's canceled (or expired). If you do not want to wait till expired contact the Embassy in Japan and ask them to cancel the visa.
Not heard of one. Let us know how it goes though as the info will be valuable to know. Good luck and hope it goes well. With many immigration items a scouting trip to ask requirements often helps.
To be honest you are not likely to owe much, if any, tax in Thailand unlessing you are remitting a lot of money into a Thai bank account thst has not had tax paid on it at source. I doubt Thai tax authorities woukd ever come looking for you. This is probably more so now they are looking like they will reverse the rules they brought in for 2024.