I agree with your possibilities.
But what if he had already bound their hands, had on gloves and used an object for SA instead of himself? There is also the possibility the perp killed them first, then committed necrophilia, once again with an object. This happens frequently in deaths by strangulation.
We cannot assume there was no SA just because there doesn't appear to be a complete and useable DNA profile. We also cannot assume there was SA, but as you said, it could have been the original motive for the kidnapping and/or murders even if the perp ran out of time (and SA did not occur). I so hope it did not happen.
I understand all of this had to happen quickly.
I believe the perp had a gun or knife, with which he controlled the girls and ordered them "down the hill." He may have told them if they screamed for help or made a sound, he would kill them. At some point, I think the girls were bound and gagged, otherwise, people would have heard screams from that area during their killing.
Often, a perp will have one victim tie up the other one so they can maintain control with the weapon. When the first victim is immobilized, it is easier for the perp to then bind the second victim and then gag both victims.
Once the perp got them across the creek (over the sandbar directly below where their bodies were found) and more hidden from public view, he finished his crime. Even if the perp used a gun to control the girls, with no witnesses saying they heard a gunshot, it is more likely they were killed by knife or strangulation. In a park that had people wandering all about, a gunshot would bring them running.