It's certainly another possibility. People who take back roads sometimes do so because they are drinking. We had 3 fatalities just last night in my little community of 50,000 or so. Drinking and driving. Pedestrians get hit with frequency.
It's just weird that if RT was only 5 minutes behind her, that he didn't hear any of this.
But if I were LE, I wouldn't rule this out (and if she was just a bit off course and went further south to cross the road, then RT wouldn't have heard or seen anything).
I'm leaning toward more nefarious theories, but I appreciate that you're offering this one, because none of us has a clue what really happened.
Her body wouldn't be too far off a road, in that case, and would be found in the next year or two, I'd think. But it could be a lot longer than that...
In support of your theory: people go 60 mph or greater on Kelbaker; the road has little dips in it, obscuring other cars and would obscure a pedestrian. If she was in a hurry, she would be hard to see (mostly black and tan, except for her hat).
I think it must be very hard for LE to focus on all these possibilities, they're probably going to look at RT's digital footprint very closely (I hope, I hope they do).