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.
Respectfully snipped for focus.
So, here's the problem with doing anything in Yellowstone that involves blood: carrion creatures, including wolves. They would have attracted a lot of attention, either from looky-loos or wolf fans or wildlife cams or coyotes. And there's no telling where the cams are located.
That's not to mention carrion birds.
BUT....
All this time, I've been surprised that, per the photos, the LV gang seem to have gone into Yellowstone from Jackson Hole. The long way. I've gone through Rexburg to get to Yellowstone, and I went via West Yellowstone. It seems the most logical route to me. It's possible they made a loop trip and came back that way.
Here's the thing about that area of the park and that route. I was in a pretty pull-over rest area in woods by the river for 2 hours, by myself, no other tourists. Barely a car driving by on the road. IIRC the Park Ranger drove by at one point, but evidently to make sure I wasn't a hooligan or pitching a tent.
This rest area has a rest room.
For sure there would have been wildlife in this area, but a person freaked out by grizzlies and wolves wouldn't have felt they were an issue here.
There would have been more traffic in this area in September (I was there in October), but not busy with school kids.
An NPS video of this area is here. The Ranger is even having a bite in this very spot (7 miles west of Madison Campground):
Inside Yellowstone - Along the Madison River
SouthWest of West Yellowstone en route back to Rexburg there are also long stretches of remote National Forest, but really, LV and CD strike me as being squeamish except maybe when it comes to murder.
This scenario would also require that they disable JJ so he can't see anything and blurt it out later.