I'm new here. And probably not saying anything new but wanted to share my thoughts.
Until we got the info about doing HW late in the evening, I figured something happened to her on the way to Mom's for dinner. This would explain the car confusion. Maybe she was planning to walk there, get the car and drive home (I know her wallet was at home, but since it's only a 1 mile/2 min drive, maybe she thought nothing of not having it with her). Then when her brother saw the car was there the next morning, he texted about needing a ride, maybe figuring since she left the car, she got a ride with someone else.
But the HW thing throws me off. Of course, we don't know what "late in the evening is." And like someone else posited, "evening" could mean like 8:00 to some people, so did she do HW and then log off and go walk to her mom's and that theory is still solid? If nothing happened to her until overnight/morning hours at home, where was she walking to at 730 when the neighbor saw her if she never made it to mom's for dinner? Just going for another walk/run?