The detective murder genre, especially of popular (as opposed to literary) literature, makes no claim to realistic portrayals of events, characters or actions. Enthusiastic readers who give "rave reviews" in this genre are most often fans, as opposed to textual analysts, who are looking for thrilling entertainment rather than realistic and logical portrayals.
If you look at the text as a textual analyst (which I suppose you are learning to be), you will see that there are unrealistic representations right from the start of the novel. Setting aside the confusion between the expected gender of the initial narrator and the eventual revealed gender (the narrator sounds like a young man at the start, then is revealed to be an older woman with heart trouble), a gender voice problem Anne Bronte had also in The Tenant of Wildfell Hall, you'll find your first departures from the realistic in Margaret's narrative. Is it realistic that she would sit by the body of a killed (by accident or intent) student with whom she had a friendship of sorts and wind up feeling personal peace? No, not at all. Is it realistic that she would be tearing a path, ignoring her heart condition, to the college then allow Gregory to divert her back and drag her down to the scene of the death? No, not at all. Is it realistic that she would suddenly turn a suspicious prophetic tone at the end of her ruminative, introspective missive (only because the author needed foreshadowing)? No, not at all.
Harkness and Dalgliesh engage in unrealistic conversation as soon as they are introduced in party with Sir Alred Treeves. Is it realistic that Harkness's question about Sir Alred's opinion of his son's vocational choice for the clergy should lead to suspicions of suicide? Of course not. All of us who have watched Midsomer Murders and Inspector Morse and The Glades know quite well that "What did you think of your son's choice?" leads not to suicide but to a quarrel between father and son, with the father cast as a significant suspect. Yet we have Harkness unrealistically making the illogical leap from "opinion of vocation" to suicide when he should know, like the rest of us and Sir Alred himself, that, having attained his objective, Ronald had no reason for suicide.
[Treeves] was rising from his chair when Harkness asked, "Were you happy, Sir Alred, about your son's choice of career?"...
"What precisely is that supposed to mean? ... He's got what he wanted."
Dalgliesh said quietly, "But if it wasn't what you wanted?"
"... [He] wasn't stupid. ... He certainly wasn't going to stick his head under a ton of sand to disoblige me."
One might say that the unrealistic is the bedrock of the premises in this popular genre novel. As to an appropriate ending, such an ending is one that follows logically--and morally--from events and actions and that is the natural and inevitable outcome of the protagonist's actions, choices, decisions, and/or revelations in the climax. As an example, a novel that does not have an appropriate ending is The English Patient because, while the theme, beginning and title promise that the English patient will be the subject of the resolution, Kip becomes the new surprise protagonist, according to the author, and monopolizes the ending. If the ending of a detective murder fulfills the requirements for logic, moral right, and inevitability from the climax, then you may conclude that it is an appropriate ending. This may require defining the major thematic statement and will require pinpointing the climax.
My suggestion is that if the crimes feel unrealistic to you, they probably are, because realism is not a concern of this genre. I suggest you analyze the text in the bits that seem unrealistic, looking for logical or chronological or motivational inconsistencies and breaks in flow (like the example above with Sir Alred). In this way you can ascertain whether they are or are not realistic. Then you can ask, whether realistic or not, "Does it matter to the genre? Does it matter to me, the reader?"