I don't have the link handy, but the showrunner has answered most of the questions presented in this thread.
A few highlights from memory...
Richard Harrow's fate...
Warning: Spoiler! (Click to show)
It's not a foregone conclusion that Margaret will be a goner. She may simply say to Nucky, "I gave you my life and saved yours. This was the cost." She knows he lied to her and will continue to lie to her. She's made a deal with the devil. He does still like having her around.
Van Alden went to Cicero, IL (just outside Chicago), where Al Capone lived during his reign of terror in Chicago.
There's no way everyone can die at any time. Many of these characters are historical characters, so the general roadmap is written. Characters like Jimmy, or any dramatic add-on, are more at risk. Nucky, Capone, Rothstein, etc. aren't going anywhere.
Those are the answers I remember to the questions I remember from this thread.