2) "Draenor" is the orc homeworld.
3) The Draenei arrive as exiles and name it Draenor which is Eredar for "Land of Exiles" or something.
4) The explanation is that orc's just called it "world" and adopted Draenor because they liked it better.
5) Seems it would be a big unifying thing for Garrosh to promote calling the orc homeworld something orcish.
So why not call it Warlords of "Gug"? Or whatever the orcish word for world is?
Much like calling Earth, Earth. Or even Terra, which is Latin for Earth.