Yes,they're certainly one of the nations under threat
Japan should act.
One could certainly argue that they have the right to act but even if they have the ability I doubt they'd do so.Why,you ask...well,consider that their military is called the "Self Defense Force".
The Defense Department used to be called the War Department. I miss the old days when spades were spades.
Japan is the only active threat. The Threat to the USA is passive and perhaps bluster.
The missiles over flew Japan. therefore a purely defensive military act would include shooting them down. It can be argued that Japan was actually attacked but that the attacking mssiles were faulty and missed.
Under treaty, if asked by Japan, America is obligated to shoot them down. If South Korea concurs, it can happen