1. Would it really have a clear end game? Look at Iraq. That may have had an ancillary link to the "war on terror" but fundamentally it was a war against a nation state. The defeat of the Iraqi military and Saddam Hussein didn't end the conflict, instead an extended insurgency ensued, followed by secretarial violence, followed by more insurgency, followed by ISIS. Iraq isn't some special case either, it happens all the time, a powerful country occupies a weaker one, they defeat the weaker country's army and government, but former military forces and civilians begin an insurgency. Also you can bet that some North Koreans will forego guerilla resistance and head over to the United States to commit terrorist acts.

2. We may have air superiority but I've only ever heard that there's no way of knowing where all of their artillery is and disabling it, and in any scenario at minimum there would be hundreds of thousands of South Korean civilian casualties from the shelling of Seol. The strength of their nuclear arsenal is debatable, but I have no doubt they could hit a nearby target such as Seoul, Guam, Japan or Hawaii. Even if all they take out is Guam that will mean hundreds of thousands dead making it the deadliest bombing in US history. I could also see them simply firing a nuclear missile into the upper atmosphere and detonating it to create an EMP that will black out South Korea, Japan and parts of China. Since North Korea has very little electrical infrastructur as it is they'll have little to lose while their neighbors will suffer dearly.

3. I highly doubt defection would be common among North Korean troops. You can't compare them to the Iraqi military. Even under Saddam Iraqis had a fair amount of contact with the outside world. Whats more by the time of the first Gulf War there still were millions of people who remembered a time when the US was an ally to Iraq during their war with Iran. During the Second Gulf War there were millions more who knew that they could safely surrender as they did in the first.

North Korea is a whole different ball game. From childhood their people are fed nothing but propaganda about how the United States is the ultimate evil in the world hell bent on destroying North Korea. War crimes from the Korean war are exaggerated beyond anything that could believably have happened. They have a museum which teaches how American soldiers drank the blood and ate the flesh of women and children. Travel outside the country is extremely restricted so almost no one has any competing information. I guarantee the overwhelming majority of their soldiers believe if they try to surrender or defect to the Americans they'll be horrifically tortured and killed. What's more their families will be punished for their defection. If you want a model for how resistant they'll be to surrender I'd look to the Soviets or the Imperial Japanese, not Iraq.