He is a very healthy looking boy to me, he looks like he is fed good food and runs around. I think it is a combination of being a little older and a photo angle that make the pictures look so different, he is doing a bit of an awkward seal pose. I am also baffled by people who see anything else but a lively expression on his last photo - it is totally the kind of photo I get of my young boy all the time.
Dad being gone could factor into any scenario in my mind - an opportunistic abductor, a known abductor or William wandering off. It is one less adult in the mix. From what I understand both women are not at their peak. I mean, if a person took him they were pretty brazen and perhaps Dad being gone meant they were willing to take a risk an altercation with the remaining women to get William, and instead a tiny window opened up when they got him away unseen.
I don't think just because the police appear baffled they necessarily are. They will be doing extensive data analysis of mobile phone towers, cctv, and monitoring any suspects, none of which they will talk about publicly until it is successful. Someone I know was contacted several years after Daniel Morcombe went missing because their phone pinged in the vicinity - let's hope it doesn't take so long, but with less people around hopefully they'll have an idea soon who was where.