The more open a floor plan appears, where there are not large support columns, the lower the weight bearing capacity will be in general.

With something like modern skyscrapers that appear to have open floor plans, there is a massive structure somewhere within the design. The structure is usually in the center built around the elevators and stairs, although there are other methods too. This structure is engineered for the specific loads of each floor and the total structure, along with various environmental factors such as weather. Like all structures, this starts with a foundation that is large enough for the designed load with a small margin of safety added. The cost of the foundation is directly related to the weight bearing capacity. No one is building structures with substantial extra unused capacity. Likewise, people like the aesthetics of open indoor spaces. This involves designing a structure with the minimal amount of load bearing capacity so that it does not need support columns throughout for the roof and upper floors. This particular aesthetic constraint means that the total load bearing capacity of department stores is very close to what you see in a typical store. If you start adding a bunch of walls and furniture to subdivide this space, there us absolutely no chance that the structure could handle the load. Even if you would like to add support columns, the foundation is engineered for the load. You can't reinforce something like this in a cost effective way. The size and depth of the pad, rebar density and structure all must be substantially different. You're likely to need piles or other features that tie in the structure to deeper bedrock elements of the underlying earth.

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.

In modern (post 70's) buildings, the interior stairwell is the emergency fire exit. New York (and some other old cities in the US) buildings are stereotypically plastered with external emergency fire stairs because they are so old and their internal stairs aren't fit (are actually a fire hazard) for emergency exit. The exterior fire escape is the exception to the building code, not the rule. They're there to compensate for the fact the buildings predate most of our modern understanding of fire and other accidents management. Modern multi-story apartments are way more resilient to fire hazards than a 1920's building, even when they are taller.

On the event of a fire you are supposed to leave your apartment and run down stairs, for which it should be: equipped with emergency lights, properly signalized to indicate exits, and use only doors with push bars towards the exterior. A well built building will have all rooms with a direct line of sight towards the apartment exit, fire alarms on every floor, extinguishers in all common areas (ideally every apartment should have one inside for the tenants), a fire hydrant or fire fighting water point in front of the building and a clearly signalized safe spot outside (distance away from the building so the risk of getting hit by something falling from the building during an earthquake is lower). The building should have fire retardant protection between each floor and between the common areas and the apartments.

This is not from any particular code, I'm no expert on any of that, just what I was taught from a firefighting volunteer friend, it is really good information to assess the safety of a building as a potential renter.