I have a question of my own, which I can't work out...
So in relation to the 'extra' body in the grave, the Thursday Murder Club were looking for someone who was around in the 1970s, who is also around now. That's what led them to John via Karen recognising his picture in the village magazine.
He told them the story about him killing the farmer, burying the body, then moving to the retirement village to keep an eye on it.
Was this all a complete fabrication, given we then find out that the body is actually that of Peter Mercer and it was Penny who killed him?
Was it just a coincidence then that Karen recognised John from the 1970s?
Did Penny bury his body there in the 1970s? Was it her who drove the move to the retirement village so she could keep an eye on the cemetery?
It seems that John, Penny's husband, had only recently become aware of what she had done given that she was in advanced stages of dementia when she told him.
John killed Ian Ventham to protect Penny, but it was obviously a very recent "decision" to do so, given he hadn't long known about Pennys past.
This is the one loose end in the book that I can't work out.