I know I sound like a broken record, but people really underestimate the ability to drive drunk (not just buzzed, but hammered) especially when you have a mission to do so (in her case getting home, because she told her mom she would).
I'm going to tell a more shaming story, instead of just saying 'i've done it' - My boyfriend lived 40 minutes away, I wasn't allowed to stay there but I could always talk my mom into another hour or two and I would always stay in touch with her. Anyway, we were all getting hammered somewhere on most weekends and I would ALWAYS drive home because I thought at the time it was the smart idea. I honestly thought it was the responsible thing to do (I was in high school - high honor roll/sports/clubs and a goal of going to a top college). There were many times I was "fine" and about 20 minutes into my 40 minute drive I was more tired and drunk than I thought. I woke up driving through a ditch, going at least 45 mph, more than once - I was VERY lucky to not hit a culvert (drain ditch with a pipe) or roll my car by jerking it car back. I considered myself skilled and smart, so I kept doing it. I was lucky to not die on multiple occasions.
I also left parties in a different direction because of sketchy people. I always had a good read on people and could tell when they were being nefarious. I didn't want to be a rape statistic, so I'd put myself in harms way to get out of harms way essentially. Late night hours change some people, especially people who have been getting messed up on who knows what.
It's really hard to say what happpened, but I do think Kiely drove her own car, by herself, into that water.