I agree with your take on this case and thought many of the same things, including the fact that the child's killer remained in the home with her, instead of taking her to another location was unusual and highly suspicious (for the Ramseys). I believe the ransom note stated an amount equal to a bonus that Mr.Ramsey's business associate never received (or some such similar scenario), and was a slick but feeble attempt concocted by the Ramsey's to implicate him.
Her death could have been purposeful or accidental, but their attempts to point the finger at an innocent to get themselves off the hook revealed their cowardice, deceptivenss,sinisterness and lack of character.
Hi!! I never heard of a business partner...who was it and what did the Ramsey's say about him, if anything.