Eh, you're listening to what Valerica wanted to believe. There's no guarantee there'd even be a war. Vampires are masters of subtlety and prefer to strike when least expected. Mortals might be more wary, but still easily overpowered. Much more easily, now that the sun isn't a factor. In any case, the feuds of minor clans would be as irrelevant as they are now. If there was a war the major powers would group together and establish treaties and pacts. And even if they didn't, there would be plenty of independent agents who would be clever enough to take advantage of the situation. My character included.
"Minor clans" is an understatement. If the sun is blocked out, the entire world will be plunged into chaos. We're not just talking about Tamriel here.
Who knows what Akaviri invaders might do when they discover the sun has been blotted out. If Harkon suceeded in turning day into eternal night, he is going to have more enemies than allies. While many vampires would prefer the darkness, many others from the provinces share the same perspective like Valerica. It is funny since the first thought my character would of had was that if Harkon blocked out the sun, the world would be turned into an apocalypse by war.
In all honesty, the Volkihar would deal with the Order Vampyrum, who conquered Cyrodiil in their own subtle way(by conuered, I mean against other clans), they would have to deal with potential armies rising to fight them off like the Empire and the Stormcloak rebellion, not counting the Thalmor and their plot to destroy the world, which in this case, might be in their favor. And the rogue dragons who fly around the skies. In no time, Tamriel would be ruins and ashes.
Come to think of it, I bet Coldharbour is what Tamriel would of looked if Harkon succeeded.