US stopped it when a much more serious enemy with ICBMs that could not be intercepted tried it, why would it lead to "crying" this time?
You're telling me there wasn't a lot of crying during the Cuban Missile Crisis?
The issue with THAAD, more than anything else, is that it pisses off the other entities in the region (namely China, but Russia by proxy as well). If it's us then who cares, they don't like us anyway. But those entities are going to be pissed at South Korea as well. And while the government welcomes THAAD, I'm not sure the people would approve it if put to a vote; many of them would prefer the US to exit the region too, I'm sure.
If all of this blows over and there is no large scale military conflict, China is not going to be happy with SK.
I know it's incredibly unlikely if not impossible, but without a 'full dress rehearsal', isn't there a teeny tiny possibility that maybe something in the real thing wouldn't work to design, when it's all put together?
I say all that to say, we'll assume NK is capable once they test the parts for a nuclear ICBM independently? We don't expect them to actually live-test a finished product?
If it's NK firing the missile then sure, anything could happen since they're still learning. But generally speaking, yes, testing them independently is good enough, just like with many other things (a car, for example, or a computer). With that said, there would be duds and the countries that have had them for decades would have a mostly functional arsenal (it's likely even a few would fail from the USA's stash).