No one should be having issues getting an apartment. Even Balmung had plenty of apartments available when I last looked a couple of months ago. It does take checking through some of the subdivisions at times. You should be able to get an apartment for each of your characters as long as they have the gil and meet the GC rank requirement.

I don't think they can set up housing so it's not tied to a specific world. Any type of housing still takes up server space and having it tied to a world helps distribute the load. They did say they were looking into trying to have houses shared by all characters on the same account and world but don't know if they found a way to accomplish that yet or if they were considering extending it to apartments as well.