Okay, I’ll bite except I don’t think it’s anybody but BM who disappeared SM.
Suspect 1]. The Stalker. He loves SM wants her for himself. Somehow he knows BM will be out of town on Sunday and that the girls are not around. Employee? Friend?
He goes to the home and she lets him in. He overcomes her so he can bring her out to his vehicle and take her away. On his way out with a struggling woman he decides to grab the bike so he can throw it over the side of the hill. He brings her to his lair where she has remained captive for the last nine months. On his way, a little farther down the road, he throws out her bike helmet to leave a trail for LE.

Suspect 2] The Intruder. He is going to rob the place. He breaks in even though there are cars and trucks at the home. He surprises SM. He rapes and kills her. Then he decides to take away her dead body and her bike.

Suspect 3]. The Stranger Abductor. He’s driving around the backroads near a busy trailhead and popular lake. He decides this would be a good place to abduct someone. He has his mind set on a cute young coed, but decides on a 49 year old woman on a bike instead. He spots her riding along the road section that is closest to the highway and the parking lot, (better chance of getting caught). He uses his vehicle to knock her off her bike and manages to do it without causing any damage. Or he jumps out, grabs her off her bike, and while she is struggling with him, he manages to throw the bike over the side of the hill. He throws her in his vehicle and drives off to kill her somewhere else. Just like The Intruder, he decides to throw her helmet out the window up the road to give LE a good clue to follow.
So,
@fcavanaugh, even though I am trying very hard to look away from BM as the main POI, I am having an extremely difficult time coming up with a good scenario that completely ignores BM’s actions, attitude, and behavior while trying to fit another suspect into the pieces of the case that we do know. Sorry!