1. No, I don't think it will. If something were to happen, I personally think it'd be a big change of some sorts. Perhaps the end of the world as we know it, but not the end of the world itself. I'm not saying I think that is what'll happen, seeing I don't think anything will happen.
2. There wouldn't be much for me to do about it if it were to happen, right? I'd probarbly just, well, run. Try to stay alive for as long as possible, helping those I care about stay alive for as long as possible as well if they were around at the time.
3. Well, yeah. If he exists, I think he would. He's not killed easily, and besides, it'd most likely just be the end of human life on this planet, in this solar system, in this universe, in this dimension. (The last part is because I believe there is more than one reality, more than one 'universe'. And it wouldn't surprise me if some overlapped in a way that's... Well, beyond human understanding (at least at this point), causing them to be pretty much alike.) I'd think Slendy could just go somewhere else, and/or sometime else if he, indeed, isn't bound by time and space. (Which I tend to believe, if he'd exist that is.)