She isn't a dreadlord... they just needed to drum up trite conflict.
Frankly it would have been MUCH easier to have the alliance leaders outraged at Sylvanas' appointment as warchief itself, seeing all the shit she pulled in northern Lordaeron. Not because of some vague misunderstanding and the horde running away because they were overwhelmed.
But again, that would require investigating Sylvanas as a character and providing Alliance/Forsaken conflict and the continuation of the Lordaeron story. Which Blizzard won't do.
I'm almost entirely certain Ashran was the reason for shifting the capitals to... well, Ashran. They really wanted to push it as a major part of content for WoD.I always felt they added Stormshield/Warspear just for PvP purposes (waiting for Ashran etc.)
Only reason a PvE-er would go there is for the portals
Well you can see how that worked out.