A close study of the plot would enable one to answer this interesting question effectively. The strength of a story and its dramatic impact is closely related to its setting and characterisation, which would definitely also affect the plot.
If one considers that Mary Maloney is the dutiful, doting wife of a police officer, one can understand why she finds it so easy to fabricate the perfect alibi to hide her crime. She would understand the ins- and outs- of police business since she would have spoken to her husband about them. She would have dutifully listened to his accounts of various criminal investigations he may have been involved in and she would have learnt. Furthermore, Mary would know that her husband's colleagues would be sympathetic to her, since their partner had been the victim of a most horrendous crime. She is therefore the poor widow who needs all their help. Mary is thus able to manipulate them. They are kind and sensitive to her and she is able to persuade them to unknowingly obliterate the evidence.
These are all factors which add to the dramatic irony and impact of the story. Mary is indeed an innocent victim of a brutal crime and since she had had only love and devotion for her husband, how could she possibly be accused of such a heinous act? Perish the thought!
Dahl paints a picture of a docile and servile wife who appears to not be able to hurt a fly. These descriptors further add to the irony and provides the story with its potent impact. Mary, the proverbial 'innocent victim', we discover, actually has a heart of stone. She does not display much remorse, but shockingly 'giggles' at the end when she realises that she is going to literally get away with murder.
The scenario would have been completely different if Patrick Maloney had been a banker. Firstly, there would have been much less sympathy for Mary from the investigators. In fact, she would have become the prime suspect. The officers would not have been as patronising and condescending as they were with the original Mary. She would most probably have been taken to the police station to face immediate interrogation. There would have been no relationship of trust and understanding between her and the officers for she would be a stranger to them.
Added to this, Mary would not have been able to get rid of the primary evidence as easily, since she would be removed from the scene of the crime. Every item would have been scrutinised twice over. There would therefore be very little dramatic irony, since all the nuances related to this aspect are linked to the fact that her husband had been stationed with these other men. The end of the story would probably be less dramatic or ironic. The fact that her husband had been a banker and thus a wealthy man, would have immediately created the suspicion of a motive for her.
In the original story, what could the motive possibly have been? Mary is pregnant, loving and caring, as meek as a lamb - just a woman dutifully taking care of her husband, nothing else. In the process she makes a gruesome and devastating discovery. Surely, she deserves sympathy?
In the alternative story, Mary could have been perceived as a woman who has grown used to the luxury and privilege of wealth, a woman who had grown greedy and impatient and was just waiting for the perfect moment to get rid of her uncaring and shallow husband so that she may enjoy greater freedom and privilege.
In this sense, the original story, because of its perfectly-timed innuendos, plot development and most importantly, its richness in irony, makes for a much stronger story than the alternative, which would have much less to offer and could present, in practical terms, only an 'open and shut' case.