I agree with alot of your points, but this is how I think Earth based on the MLPFIM universe would be like based on the races' cultures:
I always kind of got a European (Spain, France, Germany, that general area) vibe from Equestria, based on the arcitecture we've seen. Maybe the Apple family migrated overseas from America?
The Buffalo are kind of obvious, although the specific tribe we saw in show would no doubt be from the Arizona/New Mexico/ West Texas region, taking Appleloosa into account. Maybe thats where the Apples originated!
Zebras I can see being South African like you said, but with some descendants of sorts being the Carribean area if taking into account the whole slavery ordeal 1000 or so years ago that brought Africans to the west hemisphere.
Im not sure about the Crystal Empire since we've only seen a tiny scrap of it's culture and lore. I suppose Scandinavia would work since it's in the far North, Maybe Greenland or Iceland if we follow that logic.
Like the Buffalo, Saddle Arabia is kind of obvious taking the name (which is a pun of an actual Middle Eastern country) and the apperance of the delegates we've seen into account
Like with the Crystal Empire, the offical Changeling Empire was never brought into detail since it , according to the comics, was bearly established right after the Royal Wedding. I do agree with them being a sort of present-day Mongolian Empire of sorts though
The location of the Dragons are sort of world wide as a whole rather than being in the more hot, volcanic regions due to the vast number of breeds
Mules, Donkeys and Goats would mostly be in Europe alongside the Equestrians and in North American with the Buffalo.
Sea Serpents I feel would be in the Pacific Ocean with certain breeds living in the seas of the Northern Hemisphere. Like the dragons, Sea Serpents are a universal species, just in the water.
I'm not really too sure about the Griffins, Centaurs, and Minotaurs since we know little to nothing about them in the show.
These are all my thoughts atleast, though I do agree with alot of your points! I'm not trying to prove you wrong or anything but I hope my points can add some details to yours