🚀 go-pugleaf

    Therefore calls to XGetWindowAttributes or XGetWindowGeometry return -1, -1
    for the location.

I'd say you're running under a reparenting window manager, and the coordinates
are relative to the window manager's frame, not the root window.

    Can anyone say how to get the correct location of a window;

Try XTranslateCoordinates.

    how to get XConfigureWindow to work;

Should work as far as I know.

    or a good way to get subwindows to resize along with their parent?

Try using a toolkit. :-)