1. The original Aldmeri Dominion was known to have made a peace treaty with the Khajiit, the Moamer, and the Reachmen.
2. The Nords have worked with Dunmer before, specifically with Dagoth-Ur, and the Nord's desire to eatiher take/protect the Heart of Lorkhan, who is thier god-king-father, could allow them to side with Morrowind if they felt the Heart was threatened. Vivec works on god-logic and uses means and motives unknowable to other mortals and is not so vain as to not ask for help of former enemies should he need it,, and The Hist work on similarly unknowable logic and if they told the Argonians to help the dunmer, the Argonians would comply.