... We hired a building inspector to check it out and were told
that the plywood floorboards were slipping due to settling,
though the condo was structurally sound. ... Well, today I
peeled back the carpet to take a look and discovered that the
subfloor isn't plywood, it's concrete!

Are you sure that whole floor is concrete? May be the floor was wood
framed and someone put some leveling compound over the top in an
attempt to fix the problem? This means the concrete that you saw may
be the leveling compound. Does the floor sound hollow when you walked
over it? If yours is really a concrete floor, you can use leveling
compound to level the floor. Although leveling compound is expensive,
you should not need that many bags for a 10'x10' area.

Please beware that leveling compound only has limited self-leveling
capability. You cannot pour it on the floor and expect it to spread out
on the floor all by itself. You need to help it alone using a straight
edge or something. If the whole floor is basically level except for the
middle of the floor, this job should be relatively easy to do for a
DIYer. But if the whole floor is not level and if you really want to
level it, you may be better off hiring a professional to do this. The
reason is that the leveling compound is not forgiving. If you made a
mistake and you wanted to undo your mistake, you will have a hard time
removing the hardened leveling compound.

Have you paid a visit to the apartment right below yours to see if
their ceiling is OK or not? At least to confirm that the problem has
been stablized.

Jay Chan