Ok, to minimize (my) confusion I studied the original “DC3 operation procedures”. Here it is required that the MIXTURE lever must be in IDLE-CUT-OFF position before starting the engines. As soon as an engine fires, the corresponding MIXTURE lever must be moved to “AUTOMATIC RICH”. At this moment the engine only gets fuel it needs for ignition through careful priming.
From my own real experience with SEP I know that an engine can only be overprimed by priming too long (e.g. in winter), not by the position of the MIXTURE lever. The MIXTURE system is a valve, the primer is a small fuel pump (often driven manually). It bypasses throttle and mixture and injects fuel directly into the intake areas of the cylinders, regardless of throttle and mixture lever position.