Another thought I had, just regarding the weird FB comment.
I know it seems like quite a badly executed crime, but what we are forgetting is that it is only the swift actions of the family, and the fact that the LE responded to their pleas fast, that allowed this crime to be detected in the way it was.
Young people on adventures, that's what I thought when I first heard of her disappearance. I thought she would turn up, having gone on a bender, or having lost her phone. POI didn't necessarily know it was her birthday, or what her family expectations would be with regards to contact.
Maybe he saw 'backpacker' as someone who would disappear under the radar? Maybe, if he believes that, he has reason to believe it. I'm not trying to escalate or be ghoulish, but there is the possibility that he has done this before.
The crime only seems badly planned because the family were straight on it. Maybe with a few more days... he would have got away with this. The rental car had already been re-rented. The CCTV might have been deleted. IMO there was a plan in place, and the unexpected thing which scuppered him was Grace's loving family, who just had an instinct that there was something wrong. And they acted on it. Fast.