The way I see it, most people live generally where they work. The big exceptions are places like the Steamworks or the Port which require a massive labor force, which is why the "Shiftward" stations go from the commons to the Steamworks.
Wealthy individuals use portal passes to get around, meaning they can live in, say, mithril heights, but quickly get to the Market District every day.
The Market District is really just a highly commercialized district, plenty of people live there, and though many people go there to shop, it's primarily a distribution center for smaller merchants around the city.
Also, concerning Ghosts. I have a feeling some do fade away after a while. There's enough planar instability in Ishka for some to die one way or another.
The way I imagine it, Ishkan ghosts (That is, people who arise spontaneously, not because of some unfinished business or something) are held together more or less by their will. If they have something to do, they will likely stick around for quite some time. However, ghosts with nothing to do, nothing to really interest them, become more and more apathetic, just sitting and watching the world go by. This is shown by them becoming more and more transparent, as their connection to the material plane loosens. Eventually, they simply vanish.