You’re normally the font of solid common sense on here DD and there’s a lot of sense in what you say. Whatever happened that night, whether self-inflicted or otherwise, they’ve lost a child. I think you recognise that there was some element of being self-inflicted when they chose to leave their children unsupervised in an unlocked house.
I know you and that you’re a good family man who loves your kids. Parenthood is a series of calculated risks as you’ll find out more and more when your boys get older. Crossing the road on their own, letting them go to school on their own or with their pals, letting them go out at night, go away on holiday, etc. But I was a bit surprised that you’d let two non-swimmers go out in a boat without life-saving equipment.
Fortunately nothing went wrong but if heaven forbid it had,people would feel terribly sorry for you but would also say “What was he thinking?” and many would say you’d been guilty of completely irresponsible behaviour as a parent. So I don’t think you can blame people for questioning them, particularly when there’s no clear evidence of an abduction and some of their stories just don’t add up.