RSBM (respectfully snipped by me)
I think it's possible JJ was killed in the townhouse but I think Tylee would have been killed elsewhere. Mainly for the following reasons -
1. they took a trip out to Yellowstone, and Alex doesn't strike me as a hiker. I'm not suggesting she was killed in Yellowstone Park but somewhere remote on a convoluted journey home, and that was the purpose of the trip. I think it's possible Tylee was tricked into leaving the vehicle (maybe he said there were engine problems or needed to stop to see something like wildlife) JJ was drugged and sleeping at the time, carried in to bed when they got back, and given a story about Tylee going to stay at college and he would see her soon. I wouldn't be surprised to find out she was shot. If her body was dismembered I think it would be to conceal her in something, and for ease of carrying her, and Lori may have driven home with JJ and left Alex somewhere with Tylee's body, going back to get him the following day.
2. I think murdering Tylee in the house would have been a noise risk with neighbors and JJ, especially if Tylee saw it coming and struggled or JJ wandered in and discovered either the murder taking place or her dead body in the hours afterwards. I think Lori was especially vigilant of JJ when Charles was shot and that's why she waited until JJ was put in the car and had Tylee lined up to go and stop him returning to the house. I also think it risked leaving evidence there of a struggle or disturbance, like broken items.
3. I think Tylee was possibly dismembered and there wouldn't be much purpose to killing in one place and moving to another place to do that. Too much chance of getting caught with a whole body on the journey and contaminating the house and vehicle. It makes more sense to me that if one of the bodies was cut up (as I think is probable) it would be Tylee's because moving her and concealing her would be more difficult than JJ given his size and weight.
4. I think a crime scene at the house would have resulted in charges of murder and not desertion. I don't necessarily think the same would apply if JJ was killed there because firstly Tylee was already eliminated as a witness, and secondly he is not likely to have been able to put up much of a struggle against adults, especially if he was drugged, and I expect they used a means which would not cause bleeding and he could be quickly moved, leaving few traces that couldn't be cleaned up.
JMO