We should "interpose no objection" to a Japanese doctrine of strategic preemption. Especially since the ROK has become la la land regarding their evil cousins in the north.
A re militarized Japan just might help cause China and Russia to curb their enthusiasm for arming Pyongyang and snickering at us while their jackass eater throws sand at all the other kids in the sandbox.
You are correct about the ROK becoming soft and leftist leaning.
If the baloon goes up, the ROK will be about as valuable in a political sense as spit. The ROK forces are fairly reliable, but IMO the fire is not there to defend their country as long as they think the US will do the job for them, and in the present climate of appeasement towards the North.
Also, were it up to me, which you can be sure it never will be, I would move all US troops back from the DMZ to defensive positions and let the ROK troops take over the DMZ. This would get our people out of the way in case the NK's decided to stage a demonstration of infantry and artillery as a statement of agression. This would also send a clear message to the leadership of the ROK.