I still strongly suspect that Stacy wasn't the only one who called her mother that night to note the change of plans. Have suspected they also called Sherrill to let her know they were coming. When they did make this hypothetical call, the murderer was already in control or close to it; Sherrill could not/did not answer, and Suzie & Stacy left a message (speculation); killer heard the message and later deleted it, just as another message later was deleted by people arriving at the house.
The killer targeted the petite, attractive woman he'd seen earlier, probably Sherrill, possibly Suzie, possibly both (speculation); he attacked when he did because either Sherrill was a target or he wanted to get Sherrill out of the way, assuming Suzie would be back later.
He targeted at least one petite woman and theoretically invaded her home and murdered her in cold blood. If he got the opportunity to murder more petite women-- one of whom strongly resembles the original woman, only younger-- I believe such a murderer would do just that.
The potential that house would offer with its multiple access points makes me extremely fearful that this really WAS an SK. The dog factor wouldn't discourage him because that little dog sounds like he trots around quite a bit outside, anyway, and my guess is whoever this guy was-- he himself has a dog, and knows how to placate them. And he didn't just target victims, he "shopped around" for just the right combination of factors. But something brought him through town, into the area, and my guess is it was the graduation and family ties, perhaps. But it may also have been work-related, or maybe he's an outdoorsman into the Ozarks. Maybe it's a combination of these factors. Whoever did it-- agreed he's older, and for him to have actually done this and gotten away with it for the period he has... if he's not actively an SK, he's dormant, jmo.