Here's a thought: What if Alliance returns from Argus and finds Teldrassil burning? They easily believe it was done by the Horde because Alliance would never do it. The Alliance retaliate and attempt to retake Lordaeron, finding that Sylvannas already has her forces prepared (for an imminent attack on Stormwind). Alliance sieges Lordaeron and Horde defends. But perhaps we find out later, maybe from the Titan raid or Magni speaking to Azeroth that the burning of Teldrassil is really just the exit wound on Azeroth from Sargeras' final strike (but by then the war has started and there is too much bad blood to stop the juggernaut it has become). We know that Azeroth is dying so the wound must be deep; it may simply be deeper than we know. And since she is a globe, it could simply be that Teldrassil is a straigt line through her core from Silithus.