Suicide attempts in young girls, especially would probably be a "warning cry" initially, and we heard nothing about this--lots about "ADHD, etc. JMO but I strongly doubt MS took her own life. I think there would be more evidence suggesting she was considering it, more outward evidence and if she had been self harming, I think where bruises possibly by SS were mentioned by another poster, we might have seen self inflicted injuries on her arms, as an example. (I'm trying not to trigger anyone, and yes I am aware that behavior can occur on legs as well.)
Suicide is not impossible, but I think murder is much, much, much more likely. LE just needs to know what happened, and who was involved. There are many reasons that might not be clear yet, and as they have all said, SS isn't going anywhere. He's the one who had all of the horrible stuff on his phone, and he is the one who was driving around with her body in his car, so he 100% is guilty of plenty of horrific crimes.