Easy or not the increased firepower provided by an indirect fire battery was a requirement for Southern defence that was not fulfilled.

No, Ft. McAllister had the pilings sunk in the channel to prevent passage of vessels. The Mortar battery that should have been there ( and elsewhere) would have been to eliminate the military threat, not contain it.

How did you arrive at the percentiles you express the "soft" areas of the Passaic class with?

One of the benefits of indirect fire is that the battery need not be exposed to direct fire.