Over the last few days I've been contacted by a couple sources close to people involved in Maura's life in 2004, and these conversations are prompting me to consider a new theory:
In this scenario, the tandem driver delivers Maura to her safe place (a cabin, the Outing Club place, a hotel), after the crash on 112, then they return to their place, leaving her alone. Later in the week, she is found at this location by someone she knew and killed.
This would go a ways to explaining why the tandem driver is not coming forward (they honestly don't know what happened to her, and are staying quiet to protect her) but also why Maura has not been found (because she was murdered days after her disappearance far from 112).
It's admittedly a weird, messed-up scenario. But whatever answer we get will likely be equally strange.
I'm not sold on this but I'm considering it a possibility.
And if the first part of this rings true, I would ask again that the tandem driver/s come forward and give Maura's destination, because the possibility that she's alive after all of that is quite remote.