I'd just like to add my suggestion here.
Your idea of spawning bosses in Arathi and Desolace was good one to scatter people. But it's not so easy to venture into those high level zones being level1.
So, starting up with a higher level would be a nice idea(30 atleast) so that we can spread all over Azeroth and provide better data for your tests if that's what needed.
Alternatively, Limited GM command access can be given to players to level up and add mounts alone.
Or, portals to various cities can be created at starting zones
PS: Im not an expert in this, i just wrote how i felt and added my suggestion
