For what it's worth (which isn't much, lol) here's my thoughts on the boyfriend.
IMO, he didn't report her missing because they had an argument and he didn't think she was coming home. In their earlier phone conversation (where I think the argument took place) he may have told her not to come home (since she lived in his apartment) OR it could have been standard for her to stay elsewhere/leave during an argument. So, when she didn't come home, he didn't think anything of it. She may have even told him that she was staying at a friends, or a family members, and he didn't bother to call and check on her because he was angry.
We don't know what was "normal" for the two of them when they were fighting. If her stay elsewhere was common during arguments, then I could understand why her boyfriend wasn't alarmed at first.
Is it possible that her boyfriend had something to do with this? Absolutely. But, is it possible he didn't? I think that's equally, if not more, possible.
Once again, this is JMO.