For your PC, a mark and recall spell only took second to cast, and even though every Telvanni wizard probably doesn't have a mark in the middle of solstheim, I'm pretty assure there are more efficient ways of teleportation. Like the Mages Guild guides and that chick who telepots you to Mournhold. And there's also the extremely slight possibility that Aryon was kindhearted enough to port away some of the lower members to Soulstheim before tidal waves and molten lava overcame his fortress.
Looked at logically any mage who can lev is 50%+ escaped already. I'm betting the Telvanni Mage Towers survived the Tsunami as they were solid and well bedded constructions. The Mage Guilds escaped through the portals and any locals near enough to them
If true to form Hlaalu went towards the Empire while Telvanni and Redoran went North West and North East respectively.
The numbers of Redoran survivors would depend on many factors not least how many casualties they actually took against the Daedrric assault and against the Nords (not recorded) but again destroying a City is not destroying an entire civilisation. I'm sure there were more Redorans in the countryside than in the cities - and they like highlands and plateaus fine - that is the Redoran way often. Coastal settlements would stand no chance but those in upper floors of solidly built houses likely survived.
Also there are Forts to be considered. Massively built.
As I said before I'm sure that Redorans would have been seeking a new capital following the destruction of Ald-ruhn and may well have gone North. Frankly given Nord/Morrowind skirmishes I would not be surprised to learn that Redoran and the Ashlander forces had set up on Solstheim together. Counter and survival.
We will have to await Vol2 and other revelations for that.
Indoril were under intense pressure from both Hllalu and Dres - I doubt they recovered unless offshoots set up enclaves in other areas of Morrowind - likely though agreements with Redoran and Telvanni.
Dres are the dark horses. No settlements on Vvardenfell and if Hllalu were sufficiently weakened by the Imperial downturn then they would have openly pushed Hllalu in time. So likely slavery came back. That would explain Hist reaction nicely. And come the Moonfall Drres would have only their coastal settlements to lose. So likely again the Hist were aiming at Dres.
etc