There were a number of objectively conservative parties that backed Hindenburg: Catholic Centre Party, BVP, DVP, and DStP.
That's true, which means there was no possibility of a coalition being formed that involved the KPD, regardless of SPD's feelings about the KPD. Believe me, I'm not denying that the SPD hated the KPD. I'm certain of it, and it looks like so did every other party. My point is that the KPD hated the SPD, and all those other parties, at least as much. That's the thing about Marxist-Leninists, they excel at making people hate them, and they're perfectly content to be completely on their own, politically and ideologically isolated from everyone else. A plurality of German voters literally chose Nazis over the MLs. Even today, I think there are more people who hate MLs than hate Nazis, and that's saying something because A LOT of people really hate Nazis, rightfully so.