In reading the description of the home, there are two bedrooms and a bath on each floor. In my mind, I'm imagining not much being on that first floor other than two bedrooms, the bath, and a stairwell going up to the middle floor, where there are two other bedrooms, a bath, and likely the kitchen and living room. And then two more bedrooms and a bath on the 3rd level.
It's entirely plausible that the two unharmed roommates are in the first level bedrooms near the front door. The majority, if not all, of my bedroom doors in my college apartments had locks on them, just to maintain privacy when you don't always know who your roommate might be bringing home, etc. So maybe that's the case here and maybe that's why they are alive. It's also very plausible that they, too, were intoxicated. Maybe they didn't hear a dang thing and, if that's true, it's probably why they are alive. Maybe they heard something but dismissed it due to their intoxicated states, which possibly provides the estimated time of death. Or maybe they even saw something or someone come in but dismissed it due to their intoxicated state. Then maybe they remembered something in the morning and either found the scene themselves OR called someone to be with them when they came out of their room because they realized, when they were sober, that something was amiss the night before. That would explain the "a friend called 911" comment made by Ethan's mom. Regardless, I don't fault the police for keeping their identities or the circumstances of the 911 call private right now.