Because those 50 channelers alone would probably suffer casualties if they started destroying cities. People would fight back. Channelers would die (even if they surely win). Those 50 channelers would also not be able to fight the Forces of Light. Attacking cities to the south wouldn't have made...
Considering that Elayne's abilities lie elsewhere (manipulating the weather, for instance), I doubt she's got a knack for even, even if she knows the weave.
Taika: I would imagine that the Dragons arrived to late for it, and all Aes Sedai were already preoccupied with other stuff? Few could do it quickly, iirc, and none as fast as Egwene.
Perhaps cuendillar isn't unbreakable even by mundane means - they just haven't figured out how to do it, yet...