The handwriting and Rail 16 definitely link Hauptmann to the crime, but I've never been able to convince myself that he was at the window. Nor do I believe (based on the evidence I've read - the usual bits and bobs + 1 really thorough analysis of all the evidence that came out in the late 80s or early 90s) that anyone in the house was directly involved, though using the nanny or some other servant as an innocent dupe is a strong possibility.
The thing that cries out to me is the baby's death. He was dropped on his little head and buried unceremoniously in a shallow, hasty grave, fercryinoutloud. Hauptmann was a father. Everyone in the house knew the baby. The person who dropped him and buried him did not have a personal connection to him, didn't know how to handle a baby, and was cold-blooded enough to pursue a ransom demand knowing the baby was dead. That doesn't fit any of the known major players.
My reconstruction - unfalsifiable at this late date, but based on story logic - is that someone he knew approached Hauptmann for help in a criminal enterprise. Maybe Hauptmann knew details ahead of time, maybe he didn't, but he was hard up and agreed to the terms proposed. He did not, however, go on the raid, and wasn't informed of the baby's death. He wrote the ransom notes - possibly because he wrote better in English than the mastermind did - under the impression that the baby was still available for return.
So why did he take the chair instead of ratting out his evil colleagues, as you or I would do in a heartbeat? Mrs. Hauptmann and the kid. "Who wouldn't believe their husband?" She said that more than once, maintaining his innocence to the end. To a certain type of man, not disappointing that kind of faith is worth dying for. And maybe the prime kidnaper promised to take care of her. Maybe, if the cops had pressured him in the right way - "Look, we know you're just some poor schmoe who got in over your head. Give us the guy who did it. You think he'll take care of your family? He murdered that baby and he's about to let you fry, he's not gonna stick his neck out for them! Us, now, we can tell your wife any story you want, how you're the victim here. Maybe he threatened your baby. Yeah, I bet he did." - maybe this'd just be an old, closed case to us, and somebody we never heard of would be the infamous Lindbergh Kidnaper.
