This is a very confusing case, every theory has major problems.
Theory 1- The SIL did this. This is statistically favorable, most people who are victims of violent crimes are injured by someone they know. We know she arrived at AG's home from the Uber driver. We know she came back to the home because the pacemaker pinged there and her possessions were there, hearing aids, phone (which she would have needed for the Uber), keys, wallet and her blood is there, in addition to the camera going offline and face detection. For the SIL to have done this he would have needed to return undetected in the middle of the night, killed his MIL, hid her body, then returned home all while leaving no digital trace and not alerting his wife (or she could be his accomplice). The timeline seems to suggest someone who was comfortable at the site and may have had some familiarity with the layout of the house; i.e. they felt comfortable walking around and exploring, or they knew there was a back door. This seems like a story that would fall apart easily under questioning or a forensic examination of their car, phones, cameras at the Circle K etc...
Theory #2- Someone did this purposely for ransom. Statistically unlikely. The only indication of this is the ransom note but the details in it that seem to indicate the perp was at the crime scene were all available online before the note arrived- pictures of her white apple watch, google images of her home with floodlights, which almost everyone has, news reports showing the home with a damaged floodlight before the ransom note arrived, and finally no actual proof of life. The note could have been produced from publicly available information.
Theory #3- A robbery gone wrong, or a sexual assault. This is also statistically favorable, it is a not uncommon crime in around the US. The issue with this theory is that it less common to take the victim with you and it is difficult to wrangle a dead body, or a live person out of the house, especially if they are injured, bleeding, and perhaps have mobility issues. It is even more difficult to understand how you get away undetected (no traffic cams at all).