Interesting. In law school, I learned that the exact opposite way--the arsonist wasn't responsible for any damage, because the bigger fire would have consumed the target house anyway. (Edit: I believe the proper term is "superseding cause". Note that with all things involving case law, this is highly jurisdictionally dependent. For example, in California, intentional acts can be superseding causes but not unintentional/negligent acts or acts of god.)

With the mink case, the issue wasn't that blowing stuff up is a high-liability task. The issue was that it was a nuisance. Nuisance activities can be legal, and even first-in-time (like say, cow farms predating housing developments). Very nutshell version, but basically the issue with nuisance law is that the activity's effects transcend the boundaries of one's property and so the doer can be held liable for those effects.

I'll trust your memories over mine, particularly as you've studied this and I had dinner-table conversations.