We all know that the only marvel hero immune to the "fear" is Daredevil, because he is "the man without fear."  So when Daredevil saves everyone in Fear Itself, will you be happy or sad?  This basically means that Daredevil is the strongest character in the Marvel universe right?

That doesn't really make any sense. Besides as they nicely put it in Green Lantern comics: having no fear at all doesn't give you power. But overcoming your fears, that does.
But to still answer your question: I'd really be disappointed if it were Daredevil that saves the entire Earth. Not because he's Daredevil, I have nothing against the character, but because this can not be a threat that they face alone. I'm assuming we'll see several characters overcoming their fears and somehow thus overcoming all odds and kicking some Asgardian God of Fear-behind.

About you remark that if this were to happen, that this 'd render Daredevil the strongest character. No. One hit by the Hulk ( either one of them ) would cripple the man. Plain and simple.

Even if this were to play out like that......is the concept of (possible) spoiler warnings completely lost on people?

Daredevil has never been the strongest man inthe Marvel Universe.  He isn't even the 'Man Without Fear', as that now applies to Black Panther.  But more than anything, please don't create a daily 'Daredevil In Fear Itself' thread.  The one you made yesterday is ample.