3 ideas
Jardiel builds humor from situations that look impossible and absurd: relatives who never leave bed, others living as if in another decade, bizarre household rituals. He then gives each oddity a strict internal explanation by the final curtain. Laughter comes from the gap between apparent nonsense and the hidden logic that resolves it, so the absurd works as a structured puzzle rather than random whimsy.
In a household where everyone behaves strangely, one person's truly dangerous derangement stops standing out, because oddness has become the baseline. Real danger is most invisible where abnormality is normalized.
When evidence is buried and memory is controlled by the guilty party, you can recreate the original scene: the same person, the same appearance, the same setting. This forces the culprit to react to the past as if it were present. The trap targets the unprocessed guilt and fear the crime left behind, rather than the facts, so the reaction itself becomes the confession.