Location of shell casings, while seemingly important, can easily be explained away. After shooting her, OP broke into the bathroom and, in a panic, moved her downstairs. He could have easily kicked shell casings around while doing this.
Not sure if it is true, but I read that a shell casing was in the toilet. There's no way a shell casing would end up in the toilet from outside the bathroom.
Am I presuming too much to assume there are 4 bullet holes in the door? If so, a shell casing in the toilet proves that the locations of the shell casings were altered after the initial shooting (unless the shell casing ejected and bounced through a bullet hole in the door and landed in the toilet...no f'ing way).
If shell casings moved after the shooting, the defense will explain that by saying OP must have stepped on them or something. This is also how one of the casings could have ended up in the bedroom, if it didn't bounce in there from where he fired.
Obviously only my opinion, but I think OP did this on purpose (maybe in a blind rage). But I also think he is going to get off. So far the prosecution looks dumb.
This looks just like the OJ case. The exception being that he is still responsible for his actions which may lead to being charged for another crime (reckless endangerment, manslaughter, etc).