I think it's definitely possible this was a homicide. From what info has been released, there's nothing to rule out homicide definitively.
It is well known that crime scenes can be staged and not unheard of for murderers to stage a homicide to look like a suicide. beginning with the car, the animal blood could be innocuous, and have nothing to do with either theory, but it could have been placed by a murderer with the intent of misleading and confusing the investigation.
In terms of the location Cheryl was found and the causes of death, she could have been smothered at another location (maybe even her own car) and then placed facedown in the culvert. I have read that people murdered elsewhere and then placed in water can still have a little bit of water in their lungs, as Cheryl did. If the whole murder occurred in the culvert and the perp put a bag over her head and then held her facedown in the water, she could have been grasping at things in the bottom of the culvert that could have caused cuts to the fingers. As someone else said, no doctor is perfect and it's possible the finger wounds were not self inflicted. Also, if she did die in the water, most drowning victims are found facedown but in a semi fetal position, which would still be present when found due to rigor mortis. I am not sure if this was the case, they said she was found facedown but I'm not sure about the positioning of the rest of her body.
In terms of suffocation with the plastic bag, someone could have placed the bag over her head and rendered her unconscious or killed her that way without producing any injuries to Cheryl that would indicate a struggle.
I think the inconsistencies that have been discussed at length could point to a staged crime scene, specifically a homicide staged to look like suicide. The blood in the car, animal blood in the car, wallet and phone missing but glasses found, razor blade found on Cheryl, and the cause of death, are definitely red flags to me.
Any theory of homicide or suicide in this case could be proved as long as any evidence that might disprove that theory is ignored or determined to be irrelevant. I really hope the police have more information and have a forensic scientist that specializes in looking at all of the evidence and filtering through it to determine what theories of either murder or suicide do fit with the evidence.
https://books.google.com/books?id=-...M#v=onepage&q=fetal position drowning&f=false