I'm thinking if her event in HS was X-country (5k distance or 3.1 miles) she would have a 3.1 mile loop. I put together two, one from BF house, and one from Mom's. Then I virtually walked them to make minor mods for isolated stretches, or higher car traffic stretches. I've physically done the same thing for my own kids training loops.
Taking the clock-wise direction, its about 2.1 miles from the first sighting (~7:30pm) to the 2nd Street sighting (~8:00pm). At 8 to 9 minutes/mile (slower HS girl x-country) that would be ~16 to 18 minutes apart and fits the timeline better than the counter-clockwise direction. I think the last sighting is when she's walking and possibly using her phone, with the intention of continuing on to Mom's (~.7 miles).
The dogs having plenty of food and water (and in the basement) at 7:30pm to me seems too late in the evening to be their dinner. What if her intention was to run her loop, continue to Mom's, and stay the night there (not by herself at BF's). The food would then be breakfast the next day, and she could stop by to let them out and continue on to work.
I think the stretch of highway in front of the HS and on to Mom's, sometime after 8:15pm, is where something changed in her plan(?). Was she on the phone (with who) during the cooldown? Could this be the Snap to BF?