Not sure who I suspect right now, but a few things I noticed:
In the opening credits, there is a token of some kind (looks a bit like a joker) that says "BOO" on it. Then Niels is shown wearing a shirt that says "Boo" and another word I couldn't make out.
Could the title of the episode ("uitgerekend," or "calculated") refer to someone's profession rather than the first challenge? If so, it might fit Jamie best as he's the host of a science show.
In the numbers challenge, Robert pushed to ditch the height question, which seemed like a good idea (because more arithmetic was involved in that question than any of the others), but then they got to the hats question and half the team couldn't remember their own hat sizes, which I think was genuine. If he's the Mol, maybe he pushed to go in that direction knowing it would be a time-waster.
Did Merel pick up more euros in the laser challenge than she brought back? I saw that Evi did the same thing, but then she told the group about it, which is NOT a Mol-ish thing to do, so after that challenge I suspected Merel a little more than Evi (and of course Evi is now proven not to be de Mol), simply because it would be Mol-ish to take some but not all of the money and keep quiet about it. (But I'm not actually sure that she did this)
Also in the laser challenge, Sarah pulled the placard -- not the money. Could be a Mol-ish attempt to make it to the end of the challenge yet not win any money, then foiled by the fact that she lost her balance.
I don't think it's Nikkie at all. A lot of them seem to, or at least are saying they do, but she's being a little too obvious IMO. I think she was faking pretty hard at not noticing the numbers painted on the ground, for example.