I'm sorry, but I don't recall Sylvanas having any more efficient air force than the bat riders, and I highly doubt that the Horde would willingly loan her a single zeppelin for such efforts.
The bat riders cannot carry large, heavy plague tanks, only a limited stock of small bombs. Sure they may come in swarms and the bombs would be very volatile, but then again they would be shot down by Alliance air forces before they could provide any significant damage.
People seem to overestimate Sylvanas and the Forsaken as a war force. Sure, she has the val'kyr, but there's no way in hell she would ever again bring them to the front lines after the Andorhal incident, less they would get decimated above everything else as the Alliance troops would mark them as primary targets. Plus the human corpses (keep in mind the val'kyr cannot raise any other than humans) would be burned by their fellow troops, rendering them useless for reanimation.
So, no. She could not be able to conquer the whole of EK (the Horde would be very unlikely to support her), where the Alliance military force is far more powerful than hers.