Her boyfriend IIRC said he got in and noticed the cat was loopy, not as he would be if he'd spent the weekend with someone. Apparently they were planning an Xmas party early in the week so she'd been intending to tidy up, but hadn't. But he only realised her keys and phone were there when he went through her bag to see what was going on, then began to panic and after no sign of her by midnight called the police. I really don't think any of that is odd - most people don't always take a rucksack out when popping to see a friend, she could as far as he knew be elsewhere in another coat with her keys and purse. It makes sense that once it got late he began to check more and then realised something was seriously amiss.
I'm not saying he wasn't responsible. I have no idea. I thought his using the past tense to describe her when she was just missing was weird too, said as much to my DH. But then the family said they all thought she was taken and dead, so no wonder he looked and spoke as he did. If he's completely not involved he will almost certainly feel appalling guilt, however unjustified, because if he'd not gone away then she would probably have been fine. The "if onlies" must be devastating in that sort of position.
I know it's awful, and it usually is someone who the victim knows, but look at poor Joanne Lees - crucified after being the victim of a terrible crime. People often react so differently. And the fact two posters are pointing the finger, yet at totally different people, does make me think it's unfair. I mean, we can't know, can we.
Whatever the situation someone's baby has been murdered when she plainly had everything to live for and they must be devastated beyond imagining. I am so sorry for them, and so grateful that I can't understand their pain. I hope poor Joanna rests in peace and whoever did this to her is caught, and soon.