Not everything is so simple with Sasha. If the MC goes to the gym with Bella, there will come a point where he thinks he recognized Sasha from somewhere. Later in the audience, when the MC spotted the smiling eraser, the camera focuses on Sasha's face as if she know something about it. Too few facts to draw conclusions, but strange enough to go unnoticed. But you are right, Sasha's interest in the MC is mainly due to his connection with Bella, and the interest in Bella is apparently due to her friendship with Robin, who obviously has some tension in relations with Bella.
Yes, everything we have learned about this topic is still not enough to come to reliable conclusions. So far, everything we know about Summer's disappearance can be twisted to support just about any theory. Let's change my assumption from "kidnap for ransom" to "plane crash" as you mentioned earlier, and it still makes sense.
Damn, Ocean, give us an update asap, or you won't like what we make of your story!