First, it's not really a theory. I've beaten him this way many times without any complications. To keep from dying despite having low character classes, alternate between using and setting two protective dijinn such as flash and shade. This will keep your characters alive in time to use the summons you have ready and swap out new characters. It's unlikely that he will hit you with dijinn storm in the first two turns of the fight, which is the only time that dijinn storm would ruin this strategy. After that, you'll only be introducing one new character with dijinn at a time, so dijinn storm wouldn't send enough dijinn into recovery to ruin the strategy.
I have no defense for your last point, but the strategy will work well for players trying to beat Dullahan in the game.