Why not just dump.... (Spoilers)
Why not just dump.... the body in the ocean? If Ned and Maddy knew that arson was going to be detected then it would've made more sense to just dump the husband into the ocean where his body would decay quickly.
Hubby had dealings with seedy people. He also was gone for a week or weeks at a time so the authorities wouldn't have discovered him missing until the body had decayed substantially. It's also much harder to find forensic evidence of foul play in a badly decomposed body like that.