Yes, the case is the ground. But the wire going into the cap has No connection to the case, or ground.
Sure it does via the dielectric which is a high resistance. So yes if you measured it with an ohm meter you see an open or a very high resistance in the Meg ohm range. A capacitor as it charges passes current. But it won't work unless the case is grounded. When it rises to the input voltage and is fully charged it than block any additional current flow.
There is no connection between the spark plug electrode and the frame, just air. But at a high enough voltage the air conducts. But if you don't ground the spark plug it can't spark.
