They do have a tourism industry, composed of ideological "true believers" (not that big of a group), tourists wanting to experience the unique culture out of curiosity (or - in the more recent years - also to produce social media content), and tourists interested in the unique architecture.

In the overwhelming amount of cases, it also goes well enough, because it is a welcome avenue to get foreign currency for them, and despite what some may believe, the state is still fully within the dynamics of capital accumulation. (Other examples are: contractors in construction, where they have unique know-how in monumental constructions, which is also one of the places where the exploitation of their populace as wage labourers can become visible to the outside world - and selling stamps and other collectibles in demand for being "exotic" as they are from NK.)

But then, every now and again, you have cases like Otto Warmbier.

A few people have, but it's not common. https://abcnews.go.com/International/prominent-americans-met-north-korean-leaders/story?id=53635089

Yes!