Goodness knows Jane, it just seems so sloppy and badly thought out yet again to carry out such a crime supposedly planned over a year and to make so many faux pas makes me question but what do I know maybe the rifle was moved for acceptable reasons and that's where it ended up being placed before the photos..... 
Criminals are often sloppy that is how they get caught. They think things should be one way but their beliefs are wrong and this helps catch them.
If you think it is was such an obvious problem then you should be pretty scared to know the police there missed it. It doesn't make them look very good that they fell for it.
The truth is that if you shoot yourself with a rifle while lying down the rifle could land on you. Shooting yourself while lying down is unusual of course. Still the police bought that she did such because of all the other claims.