Do we even know if Japan can shoot down a missile in flight?
The last I read, even the US has virtually no chance of shooting down an unannounced ICBM.
This is false. An ICBM just means intercontinental ballistic missile meaning cross continents. Otherwise it is just a ballistic missile. The US, Russia, Israel have developed missile intercept systems. The problem is they are not foolproof. THAAD, the S-500, Iron Dome these can intercept ballistic missiles. The issue is, you can overwhelm air defense, but a single missile in place can be stopped. But remember it is also much like shooting a moving target out of air with a gun. You need to be faster than the target or have better aiming. Improved radars, better satellite positioning and imagery means, most cruise missiles and medium range ballistic range missiles stoppable. It is why Russia is developing supersonic and hypersonic cruise missiles.
Right now, the US wouldn't be able to prevent an all out Russian or Chinese missile launch. Just not possible. But a single or handful of NK missiles. Depends on the range and flight. The US can launch interceptor missiles pretty quickly, can mobilize air superiority. But to prevent say a bombing of Seoul, that is why they need they multiple sites for THAAD and MIM-104 Patriot missiles in SK.
Nope. US can only best NK with bringing in troops, attacking NK with missiles/nukes whatever while they all live underground will not end well for anyone.
That would just provoke them to do real attacks and you may even see China and Russia join in to help put down the US and anyone else who stands in their way.
What? The US can launch B2 stealth bombers in minutes undetectable by NK or China and carpet bomb all of Pyongyang unconventionally before anyone even knows what happens. Why do you think they would send troops? Are you ignoring the multiple nuclear submarines around Japan and SK right now that could probably launch multiple conventional missiles targeting most of NK without any detection? Russia and China would not challenge the world superpower if it came to NK especially if NK attacks first. Would be suicide on their end. That's what happens when you have the biggest stick. The only issue is the US no longer speaks softly.