I think in spirit, your scenario is plausible, but doesn't make the overnight bag exclusive. It's been widely reported Stacy's shorts were 'neatly folded' next to where she slept. If some crisis as such occurred, and she needed to dash outside, she had the option of her shorts to rush outside. IF that's where she was when it dawned on her what was up. Footware I'm unclear on. As for the purses, I can see abandoning that to escape danger.
Further, would Mrs. McCall know if Stacy was missing a bag ? I have never heard or read a finite point to this. Buying a swimsuit on 'the fly' for her trip is unusual from my experience. Not to say people don't do it, but I think it's more likely Stacy had the intension to stop by her house, pick-up some stuff in a few and head to Branson. We know the constant changing of plans the night before and it appears to me the Branson overnight trip was nixed by early that evening. The fact that Stacy stayed at Jannelle's or Suzie's would be irrelevant to her trip home to 'pick up some swimware and bag,' the next morning. Her mother wouldn't have known then where she spent the night. Only when things went wrong was that jig up.
The purses being left I think underscores the franticness of this dash. Not using cold, calculating logic we might, she jumped on impulse (fight or flight), grabing ANYTHING she could lay her hands on; a sheet, a bedcover, towel, whatever, or nothing at all, and left the house. I don't know if anyone has a complete enough inventory of the house to prove or disprove what's missing or not.