Preventing someone from leaving and preventing someone from leaving by commercial airline or train is not the same thing. The U.S. does a similar thing where th

Preventing someone from leaving and preventing someone from leaving by commercial airline or train is not the same thing. The U.S. does a similar thing where they might instruct airlines to prevent boarding at a foreign airport as that’s not a U.S. border and a citizen only has the irrevocable right to re-enter at an actual border. Ultimately, legal is whatever the laws of the country you’re in say plus whatever aid your embassy is able to coax out of them. That said, I don’t have answers to your specific questions about these rules.