Stanley Hazzard

Maybe they found a very good lawyer ?
And seriously, Book III was created so many years later and mostly by different people (you know, the crew) and had so many inconsistances that I think this one is just another example of them.

The reason for the inconsistency is because North & South Book II is very different from the novel it is based on, Love and War. However, and I just learned recently a major reason for the big differences between the TV series and the novel: at the time the program went into production, author John Jakes, who created the story, had not yet finished the book. Since no one knew how the story was supposed to go, the TV producers had to create their own story. But since there were major differences between Book I and the first novel, the groundwork was already laid for major changes.

Also, in the book, Stanley and Isabelle profiteered by selling poor quality shoes to the War Department... not really something that would kill anyone. Honestly, a lot of shoddy merchandise (this period is where the term originates from) equipped the soldiers and they wrote about shoes that would fall apart after one march or clothing seams that fell apart without much stress.

Iron-barrel cannon explosions were actually quite common in those days, by the way.