I don't see why this would be wrong, but when you think planet, also think plane/sphere, also think god. Nirn was created by a variety of spirits/gods/etc. (essentially those who are known as the "Aedra" now) taking parts of themselves/their plane/sphere and combining them into one.
See http://www.imperial-library.info/obscure_text/cosmology.shtml. http://www.imperial-library.info/astro/ may also have some good information on this.
That's an even better nomenclature. The world being formed by parts of the gods themselves gives Nirn a still more strange and magical nature.
That with the environment sounds good, but the races less so, most lore says all humans descend from the north, and all elves, orcs and khajiit from one race.
Reading a bit more it seems that if the Khajiit and Orc myths are correct then we have only three main original races that branched into all races we have now: Argonians (that didn't branch at all), Men and Elves.
You're all overthinking it.
The Velothi Mountains shield in the ashstorms of Vvardenfell, thus altering the flora and fauna of Morrowind. I'll find the post/text and link it if need be, but I'll hope you trust me.
That would be a real good and natural explanation, still the insectoid/mushroom based life forms of Morrowind are quite alien and strange.
But to be fair something similar ocurred in South America, where marsupials dominated the continent (like in Australia) until the Panama istmo rose from the ocean and other invading mammal species overcame them and took over the place.