I really don't get the Greenland thing. Is there some 4D chess reason for it? The US already has military bases there and could probably have as many more as they wanted if they just asked nicely. So the whole security thing is a pretext
IMO the reason is that he is a senile old man lashing out at the world. I really don't think there's a lot to it beyond that. Everyone else goes along with it because they know the administration's entire legitimacy comes from Trump and if/when he's gone the whole house of cards will collapse. That or they have their own focuses (e.g. Stephen Miller) and are happy for Greenland to be a distraction while they get on with their own stuff.
No one's really put out a strong case for it like that. You could argue it's a lot of land that's underpopulated and might be valuable in twenty years with global warming or something?
But the simpler answer is that Trump seems to personally like the idea of adding a big landmass to the US for ego reasons. He talked about it last term too with the same excuses mostly.
if there were more valuable reasons, talking publicly about it would only drive up the price.
There have been mumblings about mineral wealth, and about strategic bases. None of them really make a lick of sense.
It does not look as if it's serious. But this is a deeply unserious administration, so it's hard to guess.