Sorry for answering a question directed at someone else but I've had a theory that had a few missing pieces until the Long Beard's pings, the Wal-Mart stop and purchases came to light at the trial. Then everything fell into place. I hope it is okay to use your question as a jumping off point.
TM & SM kept trying to suggest that Heather was depressed and tried to make it seem that Heather harmed herself or because of depression just took off.
Here is my theory:
They planned this for a long time. The pregnancy test and cigar were essential to help their plan go smoothly. I'm also wondering if the pizza in the bathroom was brought by SM that night, along with the "celebratory" cigar and pregnancy test.
Is there a pizza place close to the Wal-Mart or payphone? Or in the gas station where the payphone was located?
SM orders the pizza in person so he can use any name and as with the Wal Mart purchases he uses cash, possibly he dropped TM off to order the pizza while he picked up his items. Then picked her up after the phone call?
SM speaks to Heather and tells her he's left TM after hearing Heather is pregnant. He's missed her. Can he stop by with a pizza and a cigar so they can celebrate. And a pregnancy test for them to keep as a memento! Heather trusts SM and thinks she is safe with him so invites him over.
Heather calls Bri, since Bri doesn't approve of the relationship and Heather has moved forward Heather thinks it's best to not share everything that she and SM talked about. She doesn't want to argue with Bri but does want Bri's advice.
Soon after she hangs up with Bri SM shows up with pizza, pop and Wal Mart purchases in hand. Take the test first so we can celebrate! (this way the test is taken by heather). I'll pour the soda! (SM puts something in soda to incapacitate Heather). He waits until the drug works. Then TM comes in from the truck to assist SM in staging the apartment.
They place the pizza in the bathroom, so it looks like she's eating while taking the test, tissues on the sofa, depressed posts on line using her phone. The test is left either in the bathroom garbage or her car.
(Ugh, I hate thinking this following part) Either SM or TM goes back to the payphone area to wait for the signals to head home and turn on SM's phone. The soda bottle and Heather's glass are taken along with Heather to Long Beards in Heather's car by either SM or TM where Heather is murdered and placed in the garbage. They start heading back to Heather's apt. but find they forgot to dispose a piece of evidence so head back to lb. There is one more look over Heather's apartment to make sure they haven't left anything behind and that everything fits their narrative.
This is just a theory, but gives a use for the preg rest.
Heather, I hope you are happy in the sunshine of God and let your and God's light illuminate any clues left behind, let any trace of your earthly self be found and let everything lead to the conviction of your killers.
I really think if they did not search Long Beard's and the dumpster with cadaver dogs they might want to.
