I'm actually startled that the argument here is where one side says
"What if the Union was the one to fire first?"
And the other is
"They wouldn't be."
Is it not possible that some hotheaded Union officer kicks things off by firing on a "Reb"?
Actually the OP is the following
Without a direct attack on federal facilities and troops, would there have been a sufficient public mandate in the North to declare war on the secessionists?
If so, what would have been the implications of a Civil War in which the Union fires first? If not, how would that have played out?
The answer to the bolded part is actually quite simple. Without a direct attack on federal facilities, they are massively resupplied and reinforced, worsening the rebel's position dramatically. Moreover, the inability and apparent total lack of desire by the rebels to clear their territory of military installations that are extremely dangerous to them, and willingness to let these installation be reinforced as potential staging points for attacks into the heart of their "nation", kills any chance of foreign recognition, the only way the rebels are ever going to win the civil war. Foreign powers are not going to side with a "nation" that shows no desire to defend itself. In short, not taking those forts ends any chance of the rebel successfully prosecuting their war, no matter who eventually fires the first shot.