In magical terms a door or doorway doesn't mean a physical door with a handle and hinges etc. It tends to mean a portal.
So I would suspect that by Doorway Jadis would be thinking of a more metaphysical contrivance.
As to the connection between the stable door and the wardrobe door.. I think thats just happen-stance. In order for the "bigger on the inside" concept to be explored, one needs a room or building to be in.. hence some sort of doorway.
Now.. if MN had featured a physical doorway into Narnia then maybe yes.. a connection could have been formed