Motel Key
So far the only question I haven't quite found a solution for is where the antagonist obtained a key to his wife's and her lover's motel room. Unless I keep missing something (I've seen the movie many times), we're left to assume that he knew how to make a key. Perhaps his "dick" helped with this.
Anyone have any ideas? Sometimes things like this happen in even the better movies; perhaps after editing, important scenes are lost, or even simply by mistake. I tend not to assume a mistake and try to find my answer in the movie.