As an actual /lawyer/fag, I've been super interested by this entire case, and how it's been unfolding.
Based on what I've read/my experience dealing with similar cases-(granted not a lot of cases are exactly similar to this, but similar base circumstances)-you have 3 ways this played out:
1.) The most probable, but least interesting, is he killed her accidently. Probably 75% chance of this, which I'll expand on in a second
2.) She died accidently either through his or her own negligence-(He drove off and left her alone overnight and she suffered a fatal accident, or she ran off during an argument and suffered an accident). I'd say 15% chance of this
3.) She committed suicide, which I give a 10% chance on the outside
I say 1 is the most likely, first from the standpoint his lawyer has taken. If her death was an accident, or she'd killed herself, the hands down play is to get an affirmative defense going day 1. You as the lawyer wanna be in front of the media telling them how your client is cooperating with police, you'd help him craft a written statement about all the details you know the cops are going to ask about, and you'd have done a controlled interview with counsel present. What his lawyer HAS done is all the shit you'd do when you know your client is fucked.
The body cam video from the incident in Moab is also telling. Girl has clear signs of long term emotional abuse and instability, and the boyfriend doesn't come off as being any more stable tbh. Situations like that can and frequently do lead to accidental deaths all the time, especially when you consider they'd been in close quarters with each other for multiple months
Also, all the police statements that "something" was found in the van means it's highly likely they found blood/tissue upon forensic examination. Given the state of the case, they wouldn't go public with it until they could tie it to or exclude the boyfriend in terms of foul play.
Doesn't look good for the boyfriend though
