Great question! I am inclined to believe that Coriakin eventually went back to the heavens and the Chief Duffer-or somebody else maybe; with luck, somebody smarter-"graduated" to being in charge. It's likely that the Duffers, if they recorded their history, always revered Coriakin and wondered if he'd ever return. 

Of course, you're asking more about their relationships in the Real Narnia and that's harder to say. It seems like King Frank is in charge there though it's possible he has underlings. It's not super clear how hierarchy works in the Real Narnia since we only see it briefly and no there is seeking justice or anything. Do we actually see any Stars there? The only one I specifically remember is Ramandu's Daughter who may not be a full-blooded Star.

(BTW, I wouldn't say we all "know" that Coriakin's relationship with the monopods is a metaphor for God's relationship with humanity. I mean, I consider it a very legitimate interpretation of The Voyage of the Dawn Treader, but the author never states it in so many words. Unless you're referring to something he wrote in a letter I haven't read.)

I would also question the use of the word "metaphor" in this situation.  One can certainly draw some parallels to that relationship, but remember that Coriakin was "sinful" being.  He was a fallen star. . . not a star at rest as Ramandu was.

Regardless, I had never given much thought to the idea that the Duffers would remain under Coriakin's rule in Aslan's country.  I never queried whether or not they remained with him until the end of the world.  Remember that, from a timeline's standpoint, there were only a few hundred years left in the world's history between Caspian and Tirian.  We know that Rillian was Tirian's Great-Great-Great-Great-Grandfather.  Seven generations is certainly less than 300 years.

Yes, I was just going to say too... I can't see any metaphor in Coriakin and the Duffers for God's relationship with humans, and I don't think I've ever read any commentary that suggests that's what Lewis was getting at. Where are we supposed to get that conclusion from? Not only is Coriakin a created being himself rather than a divine one (Aslan creates the stars in MN and there's no suggestion at all that they're somehow on his level!), but as @DiGoRyKiRkE says, Coriakin is a sinner himself in some fashion that humans aren't entitled to know, as we learn from Ramandu in the book.

Even aside from that, he certainly didn't create the Duffers himself, and he doesn't have any kind of ultimate power over their lives and their fate, so how can his relationship to them be read as anything like God's relationship with humankind? That doesn't make sense in any way I can fathom, although I'd be interested to hear a different interpretation with reasons for it.

Coriakin and the Duffers / Dufflepuds don't get a mention among the characters from Narnia's past that we meet again in Aslan's country in LB, so we can only speculate about what happened to them. I would guess that Coriakin certainly went to Aslan's country once the mortal world of Narnia ended — he and Aslan are clearly on friendly terms in VDT, so we can assume that whatever Coriakin was being punished for, he's learning his lesson willingly and definitely won't be exiled from Aslan's presence in the hereafter. Maybe his penance was completed by the time Narnia ended, although that's another good point DiGoRyKiRkE makes, that there are only a few more centuries left between Caspian's voyage and the end of the world. (I've just checked the Narnia timeline and it's actually just under 250 years.) 

We also don't have any indication of what may have happened to the Dufflepuds over that last period of Narnia's history. If Coriakin finished his time on earth and left them, they might have set up their own society in their own oddball way, with their Chief or his descendants ruling over them. Or Coriakin may have been with them until the end. We don't know what their view of Aslan might have been, since he himself chooses not to appear to them, explaining with what seems like a laugh that he would "frighten them out of their senses" if they saw him. But he (Aslan) does seem to view them with amusement and affection rather than disdain, and I would guess that when each of them died, they would be brought before Aslan, and they would be welcomed into his country if they looked at him and loved him, even if they were afraid — just like all the creatures at the end of the world in LB. But there's no indication anywhere that the Dufflepuds' fates are meant to be permanently bound up with Coriakin's. Presumably they would meet him and know him in Aslan's country — and hopefully be less afraid of him then! — but I can't see any reason why they would need to continue to be his servants there.