The cartel angle has never made any sense to me. Why would a cartel that brings in $100s of millions to billions selling drugs travel to the USA to kidnap/disappear a random grandmother for a $million or even $6 million? It makes no sense. Plus, let's say it was the cartel. Those guys are criminal pro's. They would be in and out of that house in 5 minutes. They wouldn't stick around for 40 minutes or whatever it was. On top of that, let's say the cartel did it and got caught. Trump has already declared the cartels a terrorist organization and he is going to eliminate them. Why give POTUS one more reason to go after them? They aren't idiots.
What does make sense is the SIL. He was with her that night. Dropped her off. I think there is a strong possibility he discussed $$$ with her. She said no to his request. He dropped her off. Stewed about it for a few hours. Maybe had some drinks. Went back to her place to try and change her mind and things got out of control. Or he could even have went back to her place later on to end her so that he and his wife would get their share of the inheritance. To me that makes the most sense to me, and the simplest explanation.
The ransom is either him trying to buy time and give a fake narrative of what happened, or just some idiots who are trying to take advantage of the situation.