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.
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!
I don't agree this is the simplest answer as rapists don't tend to take their victims with them, dead or alive. Why risk being caught with a body and evidence in your car? And if an intruder accidentally killed her during an assault, one would assume there would be at least a fair amount of evidence in the house. There isn't, and the original story was that one - just one - bed slat was down. It appears Sandra's bed was traditional with a box spring (photos from Quincy house), so I'm not sure how a (related) small blood spray would end up under the mattress and box spring.
Keep in mind also that while Sandra's house is essentially on a dead end dirt path, it is far from isolated with a number of very nearby houses. It's the sort of location where neighbors would make of note of strange cars and noises, if only because not much probably happens there.
Personally, I don't think anything happened in the house. The police seem to agree. Could she have been abducted a gun point? Yes, for sure, but facing a boring evening at home with no car, I think it's more likely she left voluntarily and ended up in a bad situation.