I don't think anyone is suggesting anyone else was in the house on the day of the murder - as you say, the police have established no one else was. It is more a discussion of whether IS, in his increasingly emotional state in the early stages of the police investigation, confided anything in his nearest and dearest. My feeling is he could well have shared some, inaccurate, self-serving version of events, because he seems to me such a weak character he would struggle to keep his cool, remain composed and not breathe a word of anything to those closest to him. I am not suggesting any kind of collusion and I don't think anyone else is. This is speculation only - and as you say it is a moot point as only IS is in the dock and rightly so.