I don't claim to have all the answers. I said it was a 'best guess'. I appreciate there are significant holes in the hypothesis. Nevertheless, whichever way I remodel it, it keeps coming back to the 'substitute child' theory.
You are being generous when you say that many people would have asked questions. We'd like to think they would but, if anything, this charade has made it clear that people don't even want to think about things critically. If they did, all the contradictions in this story would take centre-stage, but they don't: a substantial part of the UK population won't look beyond the idea that a child is missing and see that KMH and GPM are whiter-than-white. Even if one believes they are innocent, then there are still contradictions to be resolved but nobody even goes there.
Regarding MBM dying some time prior, would people even notice ? I don't know what sort of demographic you inhabit, but in some milieux, it is perfectly possible to pass someone off as 'your child' or to claim not to have a child that you do have. I can hear the childer of my neighbour, but I have no idea how many there are. More importantly, if one were replaced by another, I don't think anyone would notice.
So, were the extended famille 'in on it' ? Not exactly. I believe they all 'knew' about KMH's behaviour and worked to shield it fron people. When this cover-up happened, they were pretty much forced to go along with it. Especially now that 12M£ has been spent, they are going to reluctantly keep the charade going, because it has now reached the stage of major criminal fraud. They don't want to continue but they are in too deep, want it to stop but at the same time, don't want to put othwr famille members under the bus to do so.