I can see any one of the three being the actual target or a combination of two. But who would know that Sherrill was going to be alone that night other than the people she spoke with on the phone? And if Stacy and Suzie came back while something was being done to Sherrill, why didn't this person wait for them to go to sleep and leave undetected with Sherrill? Suzie's blinds in her bedroom were pulled down, and the only way that happens is if she hears something or sees headlights pulling into the driveway behind her mother's car, which would mean everything happened after Suzie and Stacy got home.
This perp was watching Sherrill, jmo, he probably saw her outside working on that large piece of furniture and assumed there was no man in the house. So the antenna went up for him, then he probably noticed Suzie with "sweetr," but pure speculation, all of it. I believe this person did wait for them to go to sleep, but jmo, he is a murderer with a penchant for petite women. I don't know what to make of the blinds, but if the perp's in the house, he himself would have very good reason to be messing with the blinds.
Also, have been wondering on this: What if Stacy and Suzie actually did call Sherrill? For the longest time, I assumed they had, I mean they called Stacy's mom, why would they not call Sherrill, who wasn't even expecting them and they're heading to her house? And getting no answer, they might have assumed Sherrill had somehow fallen into a deep sleep. So... could that have happened, could they have called? Then theoretically, they left a message for Sherrill not to be shocked when they showed up? And did the killer hear it, and delete it, and was Sherrill already dead? Because how would we know where they called from, provided they actually did call? They were hopping around from party to party. They could've called from anywhere, including a payphone. If LE couldn't track the obscene call they know for a fact was received in the house (subsequently message from that # erased), I doubt they could track this hypothetical call made by Stacy and Suzie, either (if it had been made). Another possibility is that the killer while inside that house saw graduation gown, cap, maybe cake in the fridge, graduation card, pics on wall... and whatever the reality was in terms of Suzie and Stacy coming there that night, he took his chances that a young female should be coming home relatively soon. He could always leave if she didn't. (And jmo, he may not even have waited inside the house, he may have returned.)
Also, evidently Sherrill's bed didn't seem slept in. It could have looked unslept in because she hadn't been in it for hours and hours by the time LE saw it.
Nothing would make me happier if all of this was flat-out wrong, and all the speculation about the women having been murdered is just--wrong. But I just can't picture that happening after all these years.