This comes down to suspension of disbelief.
Sylvanas can be killed with a single bullet ala Godfrey. If I were Greymane, I would have carried a shotgun with me and shoot her to the face right at the throne room, no questions asked.
Tyrande had her hearthstone set to SW. Unless every elf in Teldrasil has it set to Darnasus, I don't see why they didn't simply teleport out.
Characters conveniently forget stuff to advance the plot all the time. Sometimes they write convenient excuses, like some spells being too taxing, or some faction being out of natural resources to justify some war. When they don't write the excuse, we usually suspend disbelief. Maybe Sylvanas' has different kinds of screams, and her bone-breaking one was on cooldown, who knows. This only becomes and issue, I feel, when they flatten a character to the point of exhausting our willingness to care for their story.