I think it's a solid line of thinking.. Now, presuming Sherill didn't track down Suzie and the girls arrived home before she did (due to her being out at Apco), my question there is how they had time to get ready for bed, and then get into bed before Sherill had even come back. I would think her not being home would have caused some confusion for Suzie, and they would've waited up to see where she'd got to. Sherill didn't make any stop at Janelle's place or the Battlefield parties, so presumably would have come home if the tip was legit and indeed happened that same morning.
Now, had that Apco sighting come at a different hour of the morning, it might allow some time for Sherill to have gotten back to Delmar, and someone to have arrived and something to have gone down before Suzie and Stacy got back. I'm just not sure how much I buy into the theory. Would the gas station have had surveillance cameras back then, anyway?