In my opinion, a better measure of cambelt life would be hours of engine operation. So more miles accumulated in the city would mean more hours of operation >>> earlier cambelt replacement than if the same miles were accumulated on the highway at 60-80 mph.
I also don't think whether the engine is babied or not, whether it sees high revs frequently or not, will make any difference. Stress on the cambelt is mostly about the same regardless of engine speed or load.
Regarding your driving style: TDI engines are tough little engines, but they need exercise. From the description of your driving style, I don't think you are doing your engine any favors. Short trips, where the oil never gets very hot, is hard on the oil and hard on the engine. Short-shifting, keeping the revs down, allows soot and deposits to build up inside the engine, tends to keep the piston rings from seating properly against the cylinder walls, keeps the engine from achieving its potential cylinder pressure -- resulting in loss of power and poor fuel economy -- and finally it might cause the turbo vanes to get partially blocked.
Occasionally you need to take your car out for 20-30 miles at least, getting the engine hot, so that any condensation inside the engine can boil off. Do a series of hard accelerations, mainly 2000 to 4000 rpm but don't be afraid to rev higher. Take normal precautions to avoid any legal complications, of course. Exercise the engine and the turbo, they both need the exercise.