In remixing the Mror Holds, the first thing to note is that there are no dwarves in Eberron Remixed, nor any race that would be amenable to living underground. The "dwarves" of the Mror Holds are therefore humans, who live in the mountain valleys; with their focus on banking, mining, export of ore, and the manufacture of fine goods, they sound more like the Swiss to me than dwarves anyway, which is more or less how they will be presented. A kind of fractious, clannish interpretation of the Swiss.
The "orcs" of the Mror Holds don't really work with the orcs of Eberron Remixed, who are more like the hobgoblins. Then again, the orcs of the Mror Holds aren't like the orcs of the western part of the continent either; they're really kind of their own thing, so I could make them either Atlanteans left over from before the spread of Sarlonan humans, or hobgoblins (orcs) who are unrelated or splintered from those of ancient Dhakaan. I think I actually prefer the Atlanteans, because orcs in Eberron Remixed are almost always associated together with goblins, and are much more conscious of their ancient heritage, even if they do not necessarily associate with the newer nation of Darguun.
I do have some other ideas about stuff like what's at the bottom of the Goradra Gap, what's going on with Noldrunhold, etc. but since that isn't spelled out in print in the book either, I'm hold it closer to the vest so I can use it with my players as needed. And the volcanic lava flows and ashfields near the Fist of Onatar are going to be the richest source of dragonshards on the continent, which is the most important mineral when it comes to creating magical items.
The Mror Holds population remixed
Human: 74%
Atlantean: 9%
Hyperborean: 8%
Jann: 5%
Other: 4%