A few questions: Are we assuming naval-style accommodations, refugee camp conditions, "tiny-living", or more comfortable living conditions?
My initial thought was "more comfortable", since they have been there for twenty years, but I realize they'd be more like a refugee camp. Everyone rushed in when the apocalypse happened, the military happened to be there to organize things
slightly, then the doors shut for twenty years. Considering that migration is unlikely (in-universe, the known remnants of humanity are Newcastle, a military base miles away from Newcastle, a xenophobic cult/bandit group, and a small number of vagrants - everyone else is zombies), the conditions are unlikely to be pleasant.
Still, at least it's not zombies!
Can we assume additional constructions (in the walkways, on the roof)?
Definitely.
Do we have to reserve space inside the mall for long-term food storage (ie: massive stocks of food, lasting for years)?
Probably not, I imagine them having agricultural capacity on the roof. The food storage would only need to last through winter/unforeseeable emergencies, so six months tops.
What kind of basic infrastructure are we looking at; lighting, plumbing, etc?
Plumbing is possible, they may have worked out a way of purifying rainwater and distributing piping throughout the center. Lighting is definitely out, they're on candles and lanterns. Electricity is limited to a couple of gasoline generators used strictly for emergencies. Sewage probably consists of throwing it over the wall onto the zombies, which also doubles as a cathartic and recreational activity.