You're a DM after my own heart. One of the problems with the plateau idea and normal mountains is that you're selecting away most of the land mass for playing. I'm beating on the believability to give you more islands, never mind that the valleys below them might be too shallow to let your poison fog sink.
Getting a better idea of the underlying geography\geology would make this better. Most mountains I can think of form on the edges of tectonic plates - that means long island ranges not thick ones. Even Volcanoes seem to be long irregular lines. I'm leaning to something like south america but that will probably mean your islands will be further apart and more lateral.