Watch when an old mall is demolished. Look at the structure and the way the heavy equipment is used. You'll start to see the issue better. It is usually sketchy business to demolish because of the spans involved. It is more like a light bridge construction than it is a typical building. It can't be demolished incrementally like taking out bites of the structure and working across. The ones I've seen drop the roof first and clean up what remains.

Only an engineer can really say one way or another, and every building will be different. However, I'm willing to bet the load bearing structure is specialized and unique to the application. I doubt there is any flexibility for repurposing more than 10-15% outside of the tolerance you see utilized. It is like the question why can't a Honda Civic perform like a Bugatti. Technically anything is possible if you point your money machine gun at it in a currency downpour, but is it still a Civic when a Bugatti is cheaper?

The smarter option is likely to demolish the light structure of the department store and build a proper apartment building that is then attached back onto the mall corridor.