Do you have a PDR(paintless dent repair) shop around there? If not what I would do is drop the headliner down a bit get youself a fairly hard rubber malet and a dolly or body spoon (whatever you can get on the back side of it), and work it down don't wail on it or you will strech the roof, soft taps is all you need. That will make it look better but the only way to get the roof flat agatin is to open up the paint and do body work.