Just throwing stuff out there: It could be a strategical base of operations, or shelter for Kim to retreat to, and die in.
The term "stronghold" is defined as "a place of security or survival", which is why I put that hypothesis out there.
Madmen like Kim, or Saddam, are usually egotistic. Even though they want their subjects to make sacrifices, they themselves want to survive. (s)hitler had his bunker, Saddam has his, This may be "king-dong-ill's" delusion of granduer.
I have to say that your speculation cannot be ruled out. The third underground bunker for him and his inner circle is a possibility. I was leaning to the missile base because some N. Korean defectors said that there is a missile base there.