That’s if we don’t say “sorry, we don’y have a policy that allows for leaders of enemy states to land without express permission.
This is actually the most likely outcome. Nobody would want to deal with the absolute shitstorm arresting the orange turd would cause so instead they'll just deny him entry.
The coup hasn't failed as long as Trump is still in office and his goons are still running things. Things aren't going as smoothly as the donvict wants but that's an entire universe of a difference from failing.