"he Eris/Sedna conjunction in Aries (in the charts of those over about 30) unleashed Fascism..."
I understand that you mean the *transit* of Eris on natal chart's Sedna. But I do not understand that it affects those over 30: I am 40 and my natal Sedna is in Taurus (1º16' Tau). You must mean soemthing else.
I would agree that Sedna seems to have anyhow some role as scapegoat, as it seems to be in its mythology, and in this sense it could well relate to all innocent vitims of fascism or whatever other form of authoritarian repression. But it's also from that legend, a rebel: a person who looks for her own path in life, independent from morals and societal rules. She is a searcher for freedom (anarchist? maybe).
I can just guess that Hitler's transit of Eris on natal Sedna lasted more than a few days, months or even years. I would certainly first of all look at the role (by house and aspects) that Sedna played in Hitler's chart. And for the historical interpretation, I'd look at both planets in the context of the "chart of the moment", independently from personal charts of any person.