It's because there's something cult-like about many Democrats as much as many Republicans. And yes, I do mean as much as as in equal amounts.
No. No it wasn't. Antifaschistische Aktion was a paramilitary wing of the communists, fighting both the social democratic Iron Front and the Nazi Sturmabteilung.
The only way in which it could be said to be allied with the Nazis is that both of them opposed the social democrats, but the enemy of your enemy is not in fact your friend. The KPD saw both the SPD and the NSDAP as fascists, rather than in any way allying with the NSDAP against the SPD.
Exactly. Blame/credit (blame in this case) doesn't travel that way.
Take the following example: Alice and Bob both support view X. Bob also supports view Y. Y is evil. Then, Bob can be deemed responsible for supporting view Y. But X does not become evil because Bob is. And so Alice is completely fine.
I mean yeah, he is, but saying it's new isn't why he's talking bollocks. There was an Antifa that the modern antifa claims ideological descent from, and that is what he is claiming supported Hitler.