I can't imagine what Oak could possibly have been doing down there that would have lulled security into just letting an unknown seismic even slip by. Truth be told, they could just have easily caused the cavern system to collapse. It would be kinda hard to miss if the mansion ended up in it's own sub-basement... Multiple doors or no, that shockwave would spread outwards into the adjoining areas through the bedrock. But, it is a game afer all.
As to scale, The map isn't gridded out, so there's really nothing to go by.
Stopped by the Library and actually read some of the things available. Interesting. As to Will's progeny taking over, why not a prequel taking things back to the flood and it's aftermath, letting a player "get in on the ground floor", when it all starts?