Watched this recently, I too, even though I haven't read the book in years, was wondering why they deviated so much from the plot. I'll agree that in seeing it again that O"Halloran (Farewell My Lovely 1975) is the superior Malloy, thought I do like the "Pepper's Ghost" entrance of Malloy that is employed in this interpretation, the Richards film is superior and I believe more faithful to the book.
Here are some screencaps from "Murder My Sweet"
The Pepper's ghost Malloy intro:
This version ties up the loose ends in a beach house rather than on the gambling ship and the Burnette character is absent. It also has a lame epilogue catering to the female audience.
Powell is great as Marlowe pretty much as I pictured him in my minds eye as I remember the book (Mitchum was just a tad too old and a tad too iconic, unfortunately), and I'll go with Rampling also she had a devious look in her eyes, Trevor wasn't as believable to me she played it a bit to "upper crust", all in all though, I prefer all the rest of the actors in the Richards version. 8/10