N. Korea does not need (or even want) legitimization from the US for internal control. They already control everything without any real resistance (on a state level).
Of course they do. They've used the external threat rhetoric to keep society in check for years. This isn't about whether or not there is representative political opposition in the country.
You're deluding yourself if you think the state propaganda machine isn't going to take this and turn it into state propaganda about how the US is so afraid of North Korea's nukes that they're willing to meet with Kim, stop military exercises, and withdraw their nukes.
No need to make it easier for them to brainwash the general public.
The only legitimization that could come from this meeting is in other parts of the world. But as you've written, no one else was willing to meet with N. Korea before. You used that as proof that meeting with Kim was the wrong move.
If you think that the stance of the US President means so little to our allies and other countries in the world, I don't see how you can claim he's "Making America Great Again".
If they follow Trump, then the fact that they hadn't been meeting with Kim before is no longer supportive of the fact that it was a bad idea. It's just that they were following a major political power's lead in their stance. If they hold the line, then Trump has done nothing to legitimize the Kim regime around the world.
Context, as usual, is everything. The North Korean regime has been blackballed because they have committed to nuclear disarmament multiple times in the past just to back out after they've extracted some sort of gain. That's the reason why the other negotiating sides at the table were demanding actual verifiable progress first at this stage.