I think Joes game plan enabled Nick and David to bond into the two of them being definite faithfuls (and remember we don’t see everything) so by the end they were entirely sure about each other.
they also were allowed to speak between the cat vote off and the Joe so assume at that point they both became suspicious of Joe thought there were only 3 and therefore Alan became faithful by default.
and playing for charity money changed things I have no doubt had it been for actual money Nick and David may well have voted Alan off as well there just wasn’t the need
and with the money Alan’s Charity is a really worthy cause that no one is going to begrudge either as I am sure most of us at least know of someone whose lives have been affected by neuroblastoma
Also I think the other difference between Molly and Harry is their bond was between them in her eyes he betrayed her.
Nick and David went against Joe Alan was always fairly neutral with them