Just watched the end

Roaring,

There are threads on this Tommy Darmody situation elsewhere. You might find them interesting, as I think they answer your concerns. I can say briefly here that Richard's sister may have known and was around to tell Tommy. And while Richard's wife would concededly not have allowed involvement with Gillian, Tommy could have found out a lot from which he could connect the dots based on the public record, which did exist even in those days, long before you could do a Google search.

gillian was writing letters to Tommy while she was in the mental hospital. Keep in mind after Season 4 ends the series picks up 5 years later. So basically Gillian was in the hospital for 5 years. And she knew Nucky was responsible for killing jimmy

From what I remember Gillian was trading favors I think her old performance dresses to the head nurse at the asylum and in exchange she allowed Gillian to write letters

it made no sense, and it was stupid. The show should have ended with him being alive and wealthy.

Maybe they tried to shove down some stupid ideas of morale, like crime ends in death or some stupid idea. But the exact opposite would have been much better. After everybody is dead, Nucky walks out with millions and survived. No better ending needed.

...but they hung him anyway.
Hanged, Ami. Your father was not a tapestry.

I liked it. Its not that hard to believe considering he indicated he had been in contact with his grandmother.

That's basically the jist of it, Gillian writing Tommy and setting Nucky up from the inside. That Gillian's character. Always scheming till the end to get her way. Richard had respect for Nucky and so I highly doubt he told him what happened. And his wife had no reason to either. I just think it was about Gillian getting revenge on Nucky for what he did when she was so young, setting her up with that creepy Commodore, so he could get ahead in life. And the last scene of flashbacks explained that. It was sad to see Nucky die but it was inevitable. There are no happy endings in organized crime. That's what Disney movies are for.