and the Sixth House are still at large, what would happen? We know the snow demons invaded back in the 2nd era and it looks like they both had a interest in conquering Morrowind and red mountain too since the demon king Ada Soom fought with Almalexia and the Underking in the battle of Red Mountain. If they had invaded around the same time as Morrowind right when the Nerevarine got off the boat, what would had happen? Terrifying snow demons meets bizzare cult mutants and ash creatures. Would they ally with the Sixth House or would they tried to fight with them as well?

Both sides required Divine support from the Tribunal

Vivec needed to flood the entire land while Almalexia needed the help of the Underking to defeat the powerful demon king Ada Soom in Red Mountain back in the 2nd era.

The Tribunals are forced to create a large mighty magical fence to surround Red Mountain and prevent Dagoth Ur and his legions of Monsters from expanding. Dagoth Ur and his Ash Vampires also managed to drove Sotha Sil and Almalexia away.

I assume the Imperial Legions would attack the Kamal but yes, Morrowind would most likely be overrun at first.

Suddenly, the phrase "driving out the n'wah" has some extra meat.

Seriously though, the Nerevarine prophecy would have flitted by at the time (i.e. not happened) because the Kamal invasion would have prevented the Hero from arriving in Morrowind under the proper conditions. "...[W]ithout the Hero, there is no Event." Dagoth Ur wouldn't have initiated the final phases of his grand plan, therefore the Sixth House Cult would remain out of the public eye until such a time that the Nerevarine found his or her way to Vvardenfell. It would be Kamal vs the Dunmer houses aided by their God-Kings and the Imperial Legion.

In the event that the Kamal invaded half-way through the Neravarine Prophecy, I don't believe that the Sixth House and the Kamal would be friends. They might team up to take down the Tribunal and the Empire, but the Sixth House wants to impose its blighted will upon Tamriel, so they would eventually be at odds.