I've been combing the book for as much info as related to Morrowind's destruction. It's obvious the mainland has been hit pretty hard, as evidenced by this passage:
Gone the dense forest, winding rivers, and oxbow lakes, all replaced by ash-colored desert and jagged peaks. That meant they were out of Black Marsh at last, and well over Morrowind.
"Well over" insinuates that Gilm is able to determine that from the ashen deserts. Note, at this point they are still south of Vvardenfell and Vivec Crater (which is the southernmost point of Vvardenfell).
But if we take into account the fact that no other province has apparently been damaged, we can speculate on the eruption's range- Morrowind is not a very big province in itself. Most of the Dunmer cities are located around the Sea of Ghosts and Inner Sea, apart from the Dres. And we know that some of Southern Morrowind is still habitable- the Argonians have moved in somewhat.
We also can't forget that the eruption of Red Mountain has happened before (what's referred to as the Year of Sun's Death)- there is a precedent for this. Last time, I believe it ripped Vvardenfell out of the Mainland. Obviously, the province managed to survive that one as well (though that may have been helped by the lack of an invading Argonian army)
I'm going to speculate that the city of Almalexia-Mournhold may have survived (to an extent), as it is well beyond the tsunami range and the Inner Sea would have sheltered it from lava. The only danger would come from earthquakes and falling ash. The Telvanni Islands may have survived as well, but may be buried under ash.
My speculation is that the northern 3/5 of Morrowind are at best an ash covered desert, destroyed by earthquakes and ravaged by tsunamis. Vvardenfell is completely ruined, though some structures may survive- the old Strongholds predate the first eruption, and if we use that as precedent, some notable structures may still exist.
The southern parts are relatively free of ash, but shaken up by earthquakes, thus decimating the population. Then the Argonians swept in, driving the survivors north (or killing them), to other provinces or to So(u)lstheim. (It may be that the island, now a Dunmer home, has been renamed by it's new inhabitants, or something).
Plus, if we take into account the Loveletter, there may be underground settlements that nobody is aware of.