It seems like the same theories keep getting repeated. Anyone familiar with Occam's Razor knows that the simplest answer may be the right answer. If you look at solved cases, the solution is often blatantly obvious after the fact. It's not that complicated. The hard part after solving the crime is getting enough evidence to convict.
There is no evidence that this was a burglary (nothing stolen).
While there may be all kinds of theories on why SC left her house and disappeared, none make any sense, except abduction. However, there was no ransom request. And it's highly unlikely that someone of SC's age would be abducted for any other reason.
The only disturbance appears to be to the bed. The mattress was out of place and bedsheet(s) were MISSING. This seems to indicate a possible rape followed by removal of DNA evidence.
The usual suspects are people close to the victim. However, it sounds like those have been ruled out. While it's possible that this was purely random, that is very unlikely. The perp could then be someone who SC ran into "recently."
Who might have just seen SC earlier that day and decided she would be his rape victim that evening? See where this is going?
Here's the scenario. The perp works at the car shop where SC just dropped off her car. He may have overheard small talk going on between SC, her son-in-law and grandchildren similar to: "I'll take you home." "Laina will drop the boys off at your house on Friday." You get the idea? The perp learns that SC lives alone. He looks at the work order and sees her address. That evening he goes to SC's house and easily gains entry by giving her some story: "Your car is ready, and I can take you to pick it up now if you like." Door opens to continue the conversation.
The perp rapes SC or at least attempts to. Perhaps SC fought back while he was on top of her in bed. Perhaps he then decided to strangle her even though he had no prior intent on doing that. Perhaps he wore a face covering that came off and SC blurted that out, "You're one of the guys from the shop." Now his crime has escalated from rape to homicide. Fast, think!
How to get rid of the evidence? Remove the bedsheet which has his DNA. Remove SC's body which also has his DNA. This guy is smart enough to realize that to make the house not look like a crime scene, he needs to also grab SC's shoes and purse to make it look like she left the house. This way, when she turns up missing, it will appear that something happened to SC away from the house. This is exactly what happened with the police. They did NOT label the house as a crime scene. Hence, all the crazy theories about running away, suicide, etc., none of which make sense.
I'm sure the detectives looked at this type of scenario or variations thereof. Why no arrest? Not enough evidence to convict. If they haven't considered the perp possibly being from the car shop, shame on them! They should read this thread. QED
I look forward to comments, feedback, and holes in this scenario.