The SPD initially prefered to work with further left forces. They worked together on the Reich Congress of Workers’ and Soldiers’ Councils however the SPD wanted a parliamentary democracy and the USPD wanted a council republic, so when they realized the most of the councils were not in fact communist and actually supported the SPD, that caused uprisings against the interim SPD lead government, which the USPD left. The USPD was also unwilling to work with the SPD in the national assembly, which was the parliament they set up and they were sitting in. Intresstingly the Weimar constituion has a few points which could have been easily turned to accomadate workers councils. Hence the more centrist forces worked with them and the consitution was born.

I really think the word you’re looking for here is “liberal”

No it is democratic, which the KPD at this point was no longer. They were working on setting up a Stalinist dictatorship and no longer a council democracy.

You’re making significant assumptions, such as any of the liberals actually being willing to work the with the Communists,

I am looking at what we might want to learn from what happened back for the US election and other struggles against the far right. So pointing out that this was an option is imho extremely important. Obviously they did not do it, but that does not mean it is impossible to do it at least partly today, with different left wing groups considering different centrist groups not radical enough.