The first Legion invasion during the War of the Ancients was only made possible by Azshara. The Legion didn't arrived out of nowhere. The Kaldorei were pretty together at the point and only Azshara and the Highborne were going against the rest, using more (and too much) arcane magic that it did upset Wild Gods like Cenarius, who was otherwise pretty happy with the unity of the Kaldorei with the nature of the world.
Also, the War of the Ancients didn't last for thay long. I couldn't find an exact timeline but maybe a few months at the most, with the initial invasion by Hakkar not being noticed. The Kaldorei Rebellion was pretty effective, same with rallying the dragonflights and the Wild Gods. I honestly think they were pretty effective in pushing back the Legion. They already had a military force used for fighting the trolls all accross the continent, meaning they were prepared.
I don't really understand your overall take on the Night Elfs. From what we know in the lore, they were a formidable civilization, especially before the Sundering. It's really possible they were, then, more powerful and greater in number than the Alliance and the Horde combined.