First, no, there are no international treaties about forcing aircraft to the ground. That is an extraordinary thing done in circumstances of immediate safety concerns. There are, however, treaties that have been in place about unfettered airspace transit access (no idea which exactly Belarus is a signatory to), and this was pretty clearly in violation of those. There is no "you're in our airspace so we do what we like" rule in international air travel, and that's ridiculous to suggest.

Second: they let the plane in on a flight plan that clearly took it to Vilnius. Again, that's false pretenses.

Per the analogy above: consider a consulate telling someone "sure, you can board this plane without your passport, it's fine" and then arresting the person for a visa violation on landing.