As for the Mutara Nebula/Genesis thing, you have the Deus Ex Machina explanation that the torpedo has volatile protomatter and the energy produced from the warp core explosion would provide enough energy, and probably mutate the Genesis stuff, to turn the nebula into a planet.
In other words, magic. If you blew up your computer, would it "mutate" the programming of your Adobe Reader in a way that would make it spontaneously generate the Great American Novel? That's what I'm talking about here. The Genesis torpedo was supposed to be
programmed to transform a specific target substrate in a specific way. It simply wasn't designed to work on a nebula. And you can deduce that purely from what was stated in the film itself. Diverging from reality is one thing, but when a story isn't even consistent within itself, that's simply cheating.