Its a good question. Prompts a lot of thought, but it is a bit loaded, don't you think? I don't think there are many of us that are going to answer "let them fail" for the Nuclear Reactor example (uhm.. I hope not, anyway).

Although, for the Nuclear Reactor example, I think my answer would be "call the cops". If this guy isn't your co-worker and you know enough about his project to know he's floundering, he's a national security risk.

I think the heart of your question though, is more focused on the grey area that lives in the middle. From an ethics point of view, I think that any help offered in good will is ok regardless of what it is for. It is up to the receiver of the help to take on the ethical consideration of "should I accept this help?".

From a personal reflection point of view, my natural tendency is to come on a bit strong and pushy. As a result, I try to over-compensate in the other direction. I guess my answer is exactly what you predicted... "It Depends". But the reason it depends is because I am worried about in what spirit the help will be received, not about what the help is for.