"the KPD might gain enough lower-middle class votes to form a government."
ASB.
Not really, no. For one thing, the Kaiser still being on the throne in 1919 would probably butterfly the Spartacist uprising away, which would mean that their leadership would be more competent, more willing to stand up to Moscow, & more politically viable as a result. If the KPD can actually start to grow as a party at that point without being hamstrung by Moscow's actions, then they're well-positioned to actually start breaking into the SPD's base, & if butterflies were to result in the SPD being in power when the Depression hits, then the KPD could very well have absorbed most of the SPD's leftist support. In combination with the KPD picking up enough of the Nazi's base (which would've definitely been plausible, given the makeup of the party prior to the Night of the Long Knives), then I don't think there's anything really stopping them from capitalizing on the Depression & basically pulling the same trick that the Nazis did in real life when they took control.
Now, do I think that this would've been the likeliest eventuality? Not really, no. It's certainly a plausible one, though.
