This is a pretty lucid explanation to what Roal Dahl is trying to do.. good job.
However, you made a big mistake in the end. Roal Dahl does not let the reader "create their own image of what will happen next."
If you read the end more closely, you will realise, that he mentions the tea tasted faintly of bitter almonds. Poison is known to slightly taste like bitter almonds, so Roal Dahl is trying to make us realize she poisoned him, but without ruining the mood of the story.
"The tea tasted faintly of bitter almonds, and he didn't much care for it."