This is not necessarily what I believe happened, but here's a conspiracy theory for ya:
Teresa's ex-boyfriend is jealous of Teresa's roommate. Knows about the Avery issues/lawsuit, and that Teresa makes somewhat regular trips out there for Autotrader. Maybe brings Teresa's brother into the fold for insurance? (Did Teresa have a life insurance policy, and was the brother a beneficiary?) Can also get money from Colburn & Lenk which could be another motive. The ex-bf knows Colburn or someone in the sherrif's dept. The next time Teresa goes out to the Avery's, the ex-bf intercepts her upon leaving, kills her, leaves her hidden somewhere, and notifies Colburn / Lenk. Over the next couple of days, the body is burned and the car is transported to the Avery property with some planted blood. Also, voicemails to Teresa from the ex are deleted. These are repeated calls to get Teresa's location, etc. Could have also been the ex that kept calling Teresa per her co-worker's testimony, constantly checking on her schedule and whereabouts...would have definitely been annoying to Teresa. Teresa is then reported missing, and the search centers around the Avery compound. The ex's relationship with the sherrif's dept would explain why he is so quickly put in charge of the amateur search committee. During the first day or two of the 8 day Avery compound search, the bones are dumped on the property under the darkness of night. There are too many bones, and they don't wanna make it too obvious (why would have not been seen during the first day of searching, right!) so some bones are dumped in the quarry a few miles away. The investigation continues, and Colburn & Lenk panic a little, thinking they need to add some more evidence, hence the key with only Steve's DNA, and the bullet with Teresa's DNA.
A little crazy, but not completely far fetched. Especially in this story, which already has a lot of seemingly unbelievable happenings.