True, but it sounds as though the lack of evidence of a crime has resulted in a lack of further investigation into the possibility that a crime was committed. (Although I think we can safely conclude that whatever LE took out of the house in September of 2019 didn't support the family's suspicion that something violent happened in the house. At least not to LE's satisfaction.)
Assuming Sandra made it back into her home after she was dropped off, there are only a few possibilities.
1) Sandra decided to leave on her own and walked away at some point. No one reported seeing her walking, and the area has limited walkability with a lot of nearby homes. Still, some harm could have happened to her on her journey out of the neighborhood. Self-inflicted or otherwise.
2) She walked away on her own, but managed somehow to get a ride somewhere or to the T station or to a bus and has relocated to some other place.
3) Someone stopped by with the intention of taking her and possibly harming her and she was taken out of her house against her will, conscious or unconscious.
4) Someone stopped by with the intention of just visiting with her, and they decided to go somewhere and something went wrong.
Finally, no one saw her enter her house after her Cumberland Farms visit. She was wearing the same clothing and had the same shoes and purse with her, so the possibility remains that she never made it back inside her house that afternoon.
Factors that have to be kept in mind. She had no vehicle. There were no phone calls arranging any visits to or from 47 Spoffard (aside from the allegedly unfinished plans with her cousin.) Neighbors saw and heard nothing, even though at least four homes were in closer than shouting distance.
She took nothing but her purse with her. Left her dog with no food or water in the event she was planning a long-ish visit somewhere.
If she is no longer alive, where is her body? As you point out, unless this was a truly planned abduction/murder, it can be hard to hide a body in the event of an accidental or unplanned event. But not impossible, especially depending upon the circumstances.
I just don't think we know enough about Sandra's life to fully handicap the odds of what happened to her. I think it is safe to say that Sandra had some mental health issues, although we cannot say what they might be. I would like to know if she ever had issues with drugs or alcohol in the past. Even if her daughter is fully confident that she was not using at the time of her disappearance, old demons do have a way of popping back up.