I have a similar theory, but it involves help from others (unknown friends, young friends). I'm trying to remember now why I had in my head young people and who they were, but I did write out my theory (also using Jersey Girl's timeline) somewhere in these threads! I believe in my theory Heather was dumped in the Longbeard's dumpster, though.
I'd like to know where AC's bf was during that night and the next day. I fully believe the entire family know and are involved in what happened either that night or the disposal. TM's father being 100% directly involved. Sadly i think the 2 older kids also know NOW, especially the oldest, and thats why he was willing to lie on the stand. I'm still not positive where she is but I believe she will not be found. I think the C's made sure of that in a very deliberate manner. Thats why TM is so sure of herself and cocky. She's not 'just buried' somewhere, or at the landfill. I also think something more happened in between them getting home from their trip and the meetup that night. I think SM was in contact SOMEHOW, possibly by chance, with HE and TM found out, which pushed this even further.
I go back and forth of the fact that SM was actually willing to leave TM for HE, and she found that out too... and had to get rid of the 'competition', especially if she was Pregnant. Its all about control and image for her.