better organization between Ernest Thalmann, Ernst Toller, Walther Ulbricht, Clara Zetkin, Rosa Luxemburg and Karl Liebknecht perhaps. Hold off the Spartacist Uprising, perhaps Luxemburg and Liebknecht realize the really dodged a bullet when von Baden pardoned all political prisoners and wait. Then in 1920 when Thalmann, Toller, Ulbricht Zetkin and co. form the German Communist Party Luxemburg and Liebknecht come with.
I don't know enough about Germany history to say when would be right to "spring the trap", but i think better leadership could only help the Communist cause. Also, just as the Nazis did the Communists must find a way to win the hearts of the German middle class.
I'm sure at first the SU would be ecstatic to have a Communist allie, but without doubt the brands of Communism would be divergent and polarize the Communist world between those who looked to Moscow and those who looked to Berlin for leadership. Think the Sino-Soviet conflict, just earlier.