I have had to edit your question down to focus on one question alone - multiple questions are not allowed under enotes regulations.
In this excellent short story the biggest kind of irony that is at work is situational irony. Consider how Mary Maloney is presented as the perfect wife in the opening paragraphs - she loves her husband deeply and waits upon him hand and foot:
She loved to luxuriate in the presence of this man, and to feel - almost as a sunbather feels the sun - that warm male glow that came out of him to her when they were alone together.
How surprising and unexpected, then, that the next minute she kills the object of her affection.
Dramatic irony is of course also present when we and Mary know that the police are actually eating the murder weapon whilst they are talking about looking for it:
"Personally I think it's right here on the premises."
"Probably right under our very noses."
Of course, the dramatic irony is that the police are right - for they are consuming it.
Lastly, verbal irony is also evident when Mary Maloney asks the policemen to do her a "small favour." Obviously, this isn't a "small favour" - it is actually a massive favour so she can commit the perfect murder and never be charged with what she has done.
Dahl is a master of irony and we can clearly see the three types of irony in operation in this excellent short story.