I think Occam's Razor is often overdone in these cases. We always hear, "What are the chances of it being some stranger predator", yet, Mickey Shunick, Hannah Graham, Karina Vetrano, Vanessa Marcotte, and so many many more (likely including the Delphi & Evansdale murders). There were three murders of opportunity in my town within 9 months, all women, two by the same person who did not know the victims and likely would have continued to kill, and one that remains unsolved and was done by someone spine-chillingly unhinged and it's unlikely he (she) knew the victim.
I don't think this has to be someone who knew or met Mollie, stalked her, watched her for a time, or had an axe to grind for her disappearance to make sense, and as many have pointed out, the closer (in relationship) any perpetrator may have been to her, the more likely LE would/might have someone in custody by now.