Here's a path for Comrade Hitler to the General Secretaryship:
Stalin dies/is removed from power in the 1930s (or late 1920s). The Party bigwigs hold a meeting:
Comrade 1: We need a General Secretary, one who is acceptable enough to both the Stalinists, and the others, without having a real chance to develop his own powerbase.
Comrade 2: But where shall we find such a person?
Comrade 3: What about Comrade Hitler, he's in charge of the Volga ASSR?
Comrade 4: Don't be ridiculous. He's barely fluent in Volga German, and his Russian is atrocious. What's more he's a teetotaler and a suspected vegetarian!
Comrade 2: He wasn't even born here. Come to think of it, I don't think he was even born in Germany.
Comrade 1: That's it! Comrade Hitler is perfect. He has no foundation to build a cult of personality.
Comrade 3: It'll make for good copy at the next Comintern meeting. Anyone could be General Secretary!
Comrade 4: The ideal puppet! We could control him by controlling his document translators!
Comrade 1: It's settled then! Comrade Hitler is our puppet, I mean, new General Secretary!
Narrator: Comrades 1, 2, and 3 were later denounced as MI5 agents, and killed while resisting arrest during the Night of the TT-30s in 1935. Comrade 4 became the first Soviet ambassador to the Dual State of Israel and Transjordan.