I've had this nagging feeling that Bella was drugged (heroin). Possible that MM was doing CPR and not actually hitting her in the stomach. Remember RB said her lips and face were blue and gray?
I'm reminded of the autopsy of AJ Hadsell and how the medical examiner struggled for COD. I believe there was significant research from many members who indicated the amount of time it takes heroin to escape a body under death circumstances and time. AJ was 30 days after her disappearance. Bella was found questionable months after her death after being exposed to many different elements, ie; refridgerator, water, heat etc.
Just my gut feeling but I DO think Heroin has a play in all this and they weren't able to locate it in her body. JMHO