Hard to tell. I had buy one time smaller Badmintonshoes, because it was the only pair of it and the price was pretty low. After some hard sessions I had bloody toes at the nail area and it hurts so much that I can't move on the court. For me never again.
Brands of shoes might differ in terms of size. Maybe it's the correct size. You can try it on, walk and move around and if it's comfartable and you have some space of the width of your finger, at the toe area, they will fit. Wear your sporting socks if you are at the store.
Normally the difference in length is 5mm.
I wouldn't go for smaller shoes. Especially for sports. Go for the 40.5. And if they don't sell it, order one pair.
Maybe, maybe not. It depends on material. I said that the size can differ. The best way is to try them on. If they are feeling uncomfartable don't buy them. I can't understand why the heck you want to buy shoes which are smaller than your regular size?
Never buy second hand shoes!