To fire a mortar at extremely short range wouldn't you want maximum elevation, rather than minimum, Jim?
I recall seeing a photo once of I think it was Red Devils at Arnhem. They are firing a mortar during close-up street-fighting and the tube is pointing practically at the vertical. Isn't that going to come down near you, rather than a long way off? The caption to the photo said the angle of the tube indicated how close the enemy were they were firing on.
I'm asking, rather than telling, as I have no experience of mortars in real life.