Does the Empire own/control the other provinces, or is Cyrodiil just the empire? Does the empire have forts in say, Argonia?

Yes, the Empire has control over the provinces, some moreso than others. They had forts throughout Morrowind and no doubt do the other provinces as well. Argonia may be an exception, they have forts along the perimeter but due to disease they can't go just really far inside...

Thanks

The http://www.imperial-library.info/obbooks/argonian_account.shtml gives some good information (in a pretty entertaining package, I might add) about Imperial relations to Black Marsh, if you're interested.

Sorry to bump such a minor thread, but I just wanted to thank Althesaurus for linking me to the Argonian Account. I've been reading A Dance Like Fire recently, and my only thoughts have been that it's too good for such a short read. Then what do I find but not only is the Argonian Account also written by Waughin Jarth, but that it's involving Decumus Scotti too. I had no idea there was a sequel.