1.) As for the first point, propulsion is not the same as a bomb. Yes the mechanics are the same, but what this example points out to me is that the context and concept behind an invention is an important component of the invention itself. Most people would consider nuclear propulsion and a nuclear bomb to be entirely distinct inventions, even if the mechanics are the same. And indeed there are some mechanical/physical considerations that make the inventions distinct. Blast radius considerations, for instance, are entirely different if one is constructing a bomb (in which case they might be maximized instead of minimized, etc.) as are the considerations that go into constructing the container that will house the explosion/reaction.
2.) As for point 2, Look into the nuclear fallout in Kazakstan (for which the soviet tests are responsible). Unless there have been significant developments in controlling the effects of nuclear bombing technology and more recent tests (which there may have been, idk) I would hardly think anyone familiar with the environmental side effects from those experiments would recommend it as a good landscaping tool.
One could make a similar rebuttal as the above to space applications based on the environmental impacts nuclear bombs have on earth. We don't know much (afaik) about the negative side effects using nuclear bombs in space could have, even if they're useful asteroid detonators. Radiation takes quite a long time to dissipate. This would be one reason for dissuading its use in terraforming activity, at least without extensive consideration and analysis.