If you can’t organise yourself to complete before arriving, you shouldn’t be travelling. The feckless that haven’t completed it should be made to get to the back of the queue again ( this is what Philippine immigration does).
Wrong. You’re assessable for tax on worldwide earnings according to this article irrespective of amount. 1 bn talks about a platform not an individual.
Not strictly true. If your hypothetical tax in country B Thailand would be higher than what you’ve already paid in country A, you are liable to pay the difference.