1. We also know that the guns found were the ones used in the shootings.
2. The only witnesses mentioned in the press were the young children who ran out the door past the gunman. The press has not reported whether this children identified Lanza, but often eye witness descriptions from young children, who are scared, is unreliable.
3. Its true that the motives are just speculation. Investigators may or may not have more information about motives, but the press has not really cared too much about truth.
4. Everyone is looking for consistencies between all these mass shootings, as if they are the same. The problem with both the news media, and your analysis is that mass shootings like these are idiosyncratic. They are all different -including motives, circumstances, impact of social environment, etc, etc. That's why there is no real solution to these things, and they have always happened and always will. This makes determining and understanding motives often very difficult.
5. So could someone other than Lanza have done this ? Without access to police evidence and interviews with witnesses we can't say for sure and the press is not interested in telling what happened. But, the circumstance of Lanza's body at the scene says he was at least involved. I'd bet police also have fingerprints on the weapons, ballistics from these weapons linking them to the shootings and Lanza, and lack of other evidence that would imply a second shooter.