"Everyone seems to know that Myrtle Avenue is named for William N. Monroe's daughter. But didn't he have two daughters? What was the name of the other one?"
Mabelle. The children of William and Mary Jane (Hall) Monroe were Milton Sanders Monroe, George Otto Monroe, Myrtle Mignonette Monroe, Jesse Lee Monroe (who died when he was a child, at age 7) and Mabelle Huntington Monroe. There is no Mabelle Avenue in Monrovia, however.