I'm suggesting that the communists spending energy opposing the SDP and Hindenburg for fairly valid reasons, when there were much more pressing threats to the safety and security of the entire world including themselves to spend that energy on, made their concerns about the establishment left (however valid) laughable in restrospect
This is where our disagreement is. I don't think the rise of the NSDAP was a result of the SDP splitting the vote with the KDP, I think it was the failure of the SDP and Hindenburg to address the crisis that pushed the country into reactionary politics to begin with. You could just as easily blame the SDP for not joining the KDP instead, since the KDP was reacting to the same failures of government the voters of the NSDAP were.
I especially don't attribute blame to citizen voters supporting the KDP, because not only does that not matter as much in a parliamentary system, they're also reacting to the same failures of government that the NSDAP were.
So no, I don't find that argument convincing, and I likely would not have supported the SDP given the availability of other options.
Id also point out that it was Hindenburg who appointed Hitler as chancellor. That should be evidence enough that KDP voters were right to challenge his position in the presidential election in the lead up to the parliamentary election.