[story]will scourge form by Shepard's choice ?

I believe when you board the Remnant ship later in the game trying to find Meridian, you find out that the Scourge was a weapon created by some unknown group called The Opposition to attack the Jardaan.  I don't think the blast from the Crucible had anything to do with it, not do I think that blast could have sustained the entire way across dark space to Andromeda.  That's a very significant distance..