Wasn't the problem that Hindenburg could appoint any chancellor he wanted if he rejected the pick of the legislature? By the time this election happened he had been ruling by emergency decree for like 2 years. And he was a conservative, even an imaginary alliance of centrists with the SPD and KPD would not have been able to force his hand if he didn't want to accept a nomination he didn't like, and he didn't like the socialists or the communists.