House prices will fall in the future. I can't tell you when, and I can't tell you how much, but they will fall. It shouldn't make any difference to anyone though, unless it makes it difficult to remortgage or move, because they're temporarily in negative equity.
The OP and her DH may have overpaid for the house. I've always taken the view that I'm only ever going to buy a house a tiny number of times in my life, and so I'm obviously not going to be good at buying houses, or valuing them. So I had better concentrate on the aspects that are important to me. Will the house keep me warm and dry? Will I be happy living there? Can I afford it?
I don't want to be done over, but realistically, as long as the house is right for me, then it doesn't make too much difference if I pay a bit more than the market price. In return I get the things that are important to me.
So my advice to the OP would be to concentrate on the things that are important to her. Does she really care whether they overpaid for the house? I can see that the neighbour is being annoying about it, but the problem is the neighbour, not the price paid for the house, I think. So the OP might consider what they can do to stop the neighbour from hassling them about the purchase price.
It does seem like the OP is bothered about the maintenance of the house - something that would be an issue for any house that you would buy. Here the problem is the DH, who needs to recognise that the house is a shared asset, and maintenance for the house is a shared decision to be paid for by family money. You're married. His money is family money.