After all this time, i think that Moonbox have plans for Melissa but he didn't know how to bring her back so he make Tanya take her place.
Honestly, I think Tanya might BE Melissa. Hear me out.
Tanya's family abandoned her after she revealed her pregnancy. Either they're a messed-up type of religious OR...Tanya is adopted?
As in, a little girl is recovered from a plane crash (they never found Melissa's body) and sent to an orphanage since her parents are dead.
Tanya's 'parents' adopt Melissa, rename her Tanya and raise her until she goes to the same college as the MC.
This is thin, since they should have known that they (Leonard, Anne and Melissa) had family she could been returned to.
But there could have been some kind of bureaucratic SNAFU, letting Melissa fall through the cracks.
Plus the fact that when everyone met Tanya, I think it was Liz and Amy who said she (Tanya) lookd SO muck like Allison...
I'm sure someone will pick this apart, but it's just a theory I've kicked around.
Edit: Fine,
A body was found. My mistake.
Please cite the specific place where it was confirmed to be Melissa.