About the 10" mortar fire: It is important to remember that this mortar had the opportunity to fire on the stationary USS Montauk and later the USS Passaic for ~18 hours straight in the course of several engagements (although in some of the early engagements it seems to have unsuccessfully targeted gunboats.) During that time precisely one mortar hit is recorded. In several of the engagements the monitors fired all of their shell.

The sand filled mortar shell is a less than ideal round for penetration. First, compared to a solid shot it would have about 20% less mass--reducing penetration considerably at any given range. Second, the thin wall would make it likely to shatter against even the thin deck armor. Yet this improvisation (sand filled shell) was understandable since mortar systems had not been originally intended to fire heavier solid shot. Wrought iron shot would have been the best choice from the standpoint of likely penetration.

An additional challenge for mortars firing shot vs. shell is a large increase in the wear and tear on the bed at any given range. From what I've read the CSA had more trouble producing durable beds than the Union since the Union was more easily able to adopt wrought iron beds. So going from an 88 to 120 lb projectile would likely have greatly reduced the serviceability of the mortar beds.