Hastings' importance to the Courts never made much sense to me.  Historically, it marked the end of the reign of Anglo-Saxon kings, replacing them with the Norman line.  You'd think that the ascension of the Sidhe and the declension of the Norse would coincide with the rise of Anglo/Celtic/Briton power at the expense of Norse power.  Instead, Hastings saw the elevation of the Normans (a people descended from the Norse) over the native Anglo-Saxon rulers.

Maybe the Sidhe weren't fans of the Anglo-Saxon reign, due to their ties to promoting Christianity across Britain.  Or maybe the fall of the native rulers was due to the Sidhe being distracted elsewhere.

The Battle of Clontarf in 1014 seems a much more suitable ascension point for the Sidhe, and has parallels to what theoretically happened to the Queens.  It was a battle between the native Irish and the Viking/Norse conquerors/settlers/raiders that had been harassing the Irish for a couple centuries.  The battle ended with an Irish victory, successfully repelling the Vikings from the land.  But the victory came at a high cost, with the High King and his sons all dying in battle, leaving the land divided under various rulers.

This transfer of power away from Norse Vikings would parallel their retreat from the Gates.  And if both reigning Sidhe Queens died, then it would propel the Courts into just as much turmoil as the Irish themselves were.