Assume Mary Maloney has been convicted of murder. How would the incident be displayed in the news?
I rather like the idea of this story being in the newspaper. That way a really cool, big, bold headline could be spattered across the paper. A news story on TV just doesn't quite give me that "in your face" announcement. The headline could be something like "Wife Slays Husband; Police Eat Evidence."
According to the inverted pyramid structure that you mentioned, the article is going to need to answer who, what, where, when, and why. Print space is premium real estate, so those kinds of questions are answered very quickly. The news article could start off something like this:
"Last Thursday night, Mary Maloney murdered her husband Patrick in their home. After intense questioning, Mary confessed to the murder. She claims that Patrick was planning on divorcing her, so she hit him over the head."
That takes care of who, what, where, when, and why. That last part is obviously fabricated, because Dahl leaves the reader with the impression that Mary got away with her crime.
Once your 5 w's are done, then you need to move into interesting facts and other details. You should start with the most interesting details. That's up to you, but I think the most interesting details surround the leg of lamb as the murder weapon. The news story could even use the following quote about the murder.
She swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head. She might as well have hit him with a steel bar.
From there, move on to how she disposed of the weapon.
She carried the meat into the kitchen, put it into a pan, turned on the oven, and put the pan inside.
Then this quote.
"You must be very hungry by now because it's long past your supper time, and I know that Patrick would never forgive me if I let you stay in the house without offering you anything to eat. Why don't you eat up the lamb in the oven?"
After writing about the murder weapon, write a brief paragraph about how Mary established her alibi with Sam the grocer. It's relevant to the story, but I feel it is the least interesting part of the story. Finish your news story with some kind of cheesy one liner that makes use of the Dahl's title. "Mary Maloney will be tried in court next week. She will be brought in tied and handcuffed like a lamb to the slaughter."