first one, shouldn't the material that the asteroid is made of have a big impact ? since the energy will be a function of mass and speed, the mass would be very different if the object is ice vs. stone vs. iron ... or would the difference in density be trivial at this point ? hence a 1km iron asteroid vs. a 1 km iceball will have very different energies

second, why would detonating a nuclear bomb not be effective, it seems to me four things:
(1) the detonation would give the main body of the remainder of the asteroid a nudge if it doesn't detonate right on the center (conservation of angular momentum)
(2) a big detonation should scatter at least some of the asteroid's mass across a broad enough volume so that it wouldn't hold together gravitationally (what's escape velocity for an asteroid ?)
(3) even if all of the mass reaches the earth, the fact that the pieces would have a much bigger surface area than the single object would mean that you'd have more of the asteroid vaporize in the air rather than actually hit the surface
(4) if you've got the impact scattered over a wider area, you'd have less of a possibility of getting some of the side effects, namely volcanic eruption (the single piece would have more penetrating power) or tsunami (if they landed in the same body of water then the waves should interfere with each other

what do you think ? what am i missing ?