Unique win/loss windows can't be done with more than 4 factions AFAIK. The reason MC can do it is because it REPLACES existing factions rather than add totally new ones as EF does. Unique base buildings require animations and the EF devs have a hard enough time just animating the units.
The mortar on wheels seems pretty similar to the LeIG 18 that the OH uses already, even if it's a field gun rather than a mortar.