Quote cimmimon: It all looks like Adam is the one that hit Delia, but I don't think he did it. If you have DVR, pause it right after he swerves, and you will see leaves and broken branches scattered all in the road. I believe she had been hit before he came alone and when he swerved, he could have picked up a piece of her clothing.
I think he didn't do it either. Besides what you mentioned, they focused a lot on Billy's time on his phone and Adam's car radio. When the paramedics arrived it was 8:05 and Adam was still listening to the radio. He started speeding up and he shut off the radio then he saw Dash run out. I think Billy ended up leaving him there after the paramedics showed up. Adam came around after the accident.
Adam probably came by 20 minutes after the accident.