I know I'm in the minority on this but I still feel this was done by someone who has killed before, or who at least has entertained those thoughts.
I believe she may have known him for a short amount of time, but not in an intimate way.
Some serial murderers do not sexual assault their victims because they are physically unable to do so and this would cause even more rage toward the victim. I've had many friends look at this case, even my brother in law who is a police sergeant in Detroit. They all believe it was domestic violence and I just can't see that.
The perpetrator took her shoes, her probably took jewelry and maybe a bag or purse that she carried.
These are all trophy items but also items that if disposed of properly would completely conceal her identity.
It would also distance the killer from having any connection with her.
The manner the crime was committed was calculating, like someone had thought about for quite some time and was fulfilling a fantasy.
LE won't disclose everything, but they know what was used to strangle her.
Is it an item of clothing that they're withholding from the public?
Her own underwear maybe, since there has been no mention of them?
It goes back to my first impression that she was cleaned up and dressed before being dumped.
This is all just my opinion of course.
FWIW, my first thoughts from reading a brief description about her murder was that she was an sixteen-year-old Native American runaway from an Indian reservation, who had been prostituting herself as a way of survival. I read more, and saw the better recons and post-mortem photos (in which she looks older), and then I thought: DOMESTIC ABUSE. I was having that train of thought for quite a while, until the pollen testing results came out and placed her jacket and jeans in the Southwest shortly before her murder. Made me begin to wonder if she had simply hitched a ride with the wrong guy.
I agree that he may have kept jewelry, shoes, bag/purse as a "trophy". I still feel like they were in an intimate relationship (not necessarily a sexual relationship--although that's still one of my main guesses) and it was done in a moment of rage, but he may have felt somewhat "remorseful" and taken it to "remember her by." Personally, I feel like her killer was emotionally unstable and one thing set him off; but that's just my opinion.
I remember, in Walker County Jane Doe's thread, it was being discussed that her killer may have been impotent. She had been vaginally and anally assaulted, but not through penile penetration. A bunch of people (myself included), have speculated that her killer lashed out at women due to anger over his erectile dysfunction.
About the fantasy thing; I don't know if you saw the earlier posts, but way far back, some people were speculating that her killer had braided her hair and dressed her in Native American-inspired garb to fulfill a fantasy or fetish. I think that's possible
I've wondered if clothing was used for her strangulation, too. About the underwear; since I see it not mentioned on Doe Net, I've assumed it's because she wasn't found wearing any, but I wonder if it was just not mentioned? Pretty sure it would be, but it's not impossible. She could have just been going commando, or he could have strangled her with her underwear. I feel like there would be evidence of rape or other sexual activity if he had taken off her underwear, but maybe she was out cold after being struck, if that makes sense?