I think I'd dispose of the glass outside if I could, it's just going to rip a hole in the garbage bag inside. And in hindsight, WE know what happened. In real time, if you legit arrived at a house looking for your friends, or even acquaintances, you are not thinking "let's preserve a crime scene" you're thinking "where the eff are they, we had plans!" The only red flag here is how early it was. The night after prom, and presumably either hungover or more likely still tipsy, are you up and about miles from home?
I also don't think 1, 2, and esp 3 teenagers could keep their stories straight with no breaks in the initial questioning assuming the police had any interrogation skills. More likely, if there were suspect statements, they haven't been released in the hopes of tying this up with a bow once there is more evidence.
The crux for me is that Stacey was not supposed to be on Delmar. So this either makes her part of the target because she traveled there on short notice (that FEW knew), or totally excludes her as a cause and the focus should be elsewhere. Unless the house was being watched ALL night (and/or prior), a perpetrator had no way of knowing the third car didn't belong to a Navy Seal Ninja Green Beret Special Forces Stormtrooper (you get my point). If you don't know who the third car belonged to this would be a trickier, though still feasible abduction. If you know it's another unarmed teenaged girl, you were either watching them arrive home or knew ahead of time who you'd be dealing with. Including Cinnamon. Most little dogs go batshit with a stranger, so if you were indeed a stranger, you'd need to swoop in, sequester the dog, and wrangle 3 people out without too much of a commotion. If you're armed, it would be easier, tell one of the girls to shut that dog up, but you'd still have to move quick. Three people would probably move just as you request if one had a gun to her head. And at that hour, it's entirely possible no one in the immediate vicinity heard anything.
Lastly, abducting THREE people is a feat, armed or not. It's easier to shoot three people and leave them for dead than take them elsewhere. So what motive is there to take them off premises (thereby increasing your risk of being caught) besides sexual or torture?