How can I obtain the Full Name of the currently logged in user via Terminal when run as root?
Simply use id -P $(stat -f%Su /dev/console) | awk -F '[:]' '{print $8}'

id -P $(stat -f%Su /dev/console) yields: 

adminuser:********:501:20::0:0:AdminUser:/Users/adminuser:/bin/bash


awk -F '[:]' '{print $8}' yields the 8th term (the "RealName") of an output separated by colons.

As proposed by fd0 you can alternatively use:

id -P $(stat -f%Su /dev/console) | cut -d : -f 8


which is even more simple/elegant.