they don't mind paying a toll to Iran for shipping through the Strait but won't cede authority to Iran to control which ship can sail through it (as that's against international maritime laws)

Pay up or they murder you. Is murdering people not also against the law?

Blockade is an act of war, different body of law.

I searched quickly, not an expert, but it seems the main thing is warning ships and giving them an opportunity to the captured peacefully before striking.

Not according to Hegseth and the actions of the US Dept. of War in international waters.