If Garth TRULY witnessed the vomiting, not just being told by Chad that she did, then I think that SOMEHOW Chad either poisoned her and she died of that poisoning, or he drugged her and then smothered her. If she wasn't big on doctors and traditional meds, maybe the tramadol was too much for her and she really vomited because her stomach couldn't handle it. I understand the autopsy didn't show any poison, but the ME couldn't possibly test for everything, and given the facts that we already know, the earlier attempt on her life, the way Chad openly talked about his wife dying soon, telling people she had been "so unwell", despite her sister saying she was doing great (aside from the wrist thing) the way he seemed to research EVERYTHING, his desire for CONTROL, if there was a way to poison or drug someone and have it not show up on an autopsy, Chad would have found out about it. Even though he pushed for no autopsy and a quick funeral, there was always the chance that one MIGHT be done, and I think he would have planned for that. All the time he was "writing" at the BYU library, he could have been researching on library computers every devious, evil aspect of his plan, thinking he was SO smart by not using his own phone or computer.
I JUST remembered a case I heard about where a spouse, I think it was a husband, who suggested his wife start taking a certain vitamin/supplement, he told her he saw something on the news or read an article about it, and he REPLACED the vitamins in the bottle with some kind of drug or poison and after she took it she would get so dizzy, especially while driving. Now I can't remember exactly how it played out but I will look it up right now. But my point is, one of the witnesses today described vitamins/supplements next to Tammy's side of the bed. I could TOTALLY see Chad replacing something harmless with something to make it look like Tammy was getting sicker and sicker, maybe with the plan that it would ultimately kill her, or just setting the stage for the night he finally killed her. (With Lori constantly asking when he thought she was going to die, he probably felt pressure to just hurry up and get it done.)
The reason why his/their plans never worked out perfectly is because Lori and Chad are simply not very smart.