I have been saying for years that the tandem driver theory can be ruled out with a single piece of evidence: Rick Forcier.
There's been a lot of backtracking done by certain parties to discredit Mr. Forcier, to downplay and dismiss his testimony entirely. However, the police felt strongly enough about this witness to interview him multiple times as well as search his house trailer and vehicle. Now, the latter search makes him sound like a suspect, but why would law enforcement lean so heavily on Forcier unless they believed strongly that his report of a youth on foot that night was the last known sighting of Maura?
While a lot of the online community doesn't take Forcier seriously, the original investigators did, and so do I. His sighting, while brief, happened five miles east of the crash site, thus giving the lie to the idea that Maura's disappearance occurred within some time-frame so narrow as to make foul play or death by misadventure impossible. Further, his description of her behavior (furtive, ducking down side streets to avoid being caught in his headlights) does NOT tally with someone looking to catch a ride at all, let alone with someone who followed her up there as part of a carefully scripted escape plot.
So like you, I'm left with those two possibilities. However, there is a third: that she was driving up to NH to meet somebody. That would seem to remain within the realm of possibility but for one reason: it has been nearly 12 years and this person never came forward to assist with any investigation, let alone file a report that their guest didn't arrive. Ergo, Occam's razor: this individual does not exist, Maura's reasons for going north that night were entirely personal, and Fred was correct all along.