Malls are usually multiple stories. Couldn’t you do all the plumbing on the first floor to avoid going underground. And then add structural reinforcement at the first floor to prevent higher floors from collapsing / sinking? Yeah for egress you’d have to put the bedrooms along the exterior walls. But I’m sure you could still get plenty of use over the space which does not border an exterior wall both as part of the unit and as part of some shared community area.
j4k3