One off fee. It's set at 10,000 baht, but as each embassy around the world is a separate business unit, they can set their own fees which cover their own administrative costs, so many are around the
Changes in the tax laws of individual countries do not affect the the Double Taxation Agreements between those countries. Any changes to a DTA must be in agreement with all parties. The Australian DTA has not changed since it was written in 1989 and everything covered in that agreement remain in force today
If it's covered by a DTA it most certainly 100% will prevent double taxation. The whole point of these international treaties is to ensure this doesn't happen. Why bother having them if it's not the case?