Understandable on every level, any number of things could have reoccupied. I agree that Mordheim most likely is occupied on some level, however I think you missed the thrust of the argument. A purge was conducted there and the surrounding environments, that means on some level it was cleared out of all the most obvious nasties and chaos/dhar tainted artifacts. It is a lot harder to start from scratch when you don't have the bones of whatever was there before to build upon. Not to mention with the "stronghold" of the area at least semi cleared and Stirland's armies being just across the border it is more than likely at least some level of stability has entered the region.
Now I don't mean stability as settlers or explorers or anyone sane, I mean regular patrols catch the asshats who are going to Mordheim and killing them before they arrive. Necromancers drawn to the giant wellspring of Dhar get caught by Stirland patrol's. Skaven and cults making there way get run down by Ostermark Calvary. Is it occupied and dangerous? Almost undoubtedly, but it went from "you will die if you go within those walls" to something akin to a wary sentry watching the enemy. "I know something is going on in there, but we don't know what and it's safer to catch anything trying to get in or out."
In my opinion Mordheim would require an actual army to take and occupy once again before any waystone shenanigans could be done. Yet the purge made it go from entire army of a province with help from other provinces, to an expensive, unenvious expeditionary army of Ostermark most likely. Dangerous and stupid to do without help? Yeah, but doable.
