I thought some pics might help
Here is a pic of the sagging headliner looking towards the back of the car. You can see the gap by the sunroof opening.
Here is one of the broken plastic pieces. There are two of these. The bolt goes through the center of the plastic piece and goes into the roof. This is all covered by the headliner fabric and there is no way to unscrew the rest of them that are still attached.
Okay, one last one just for fun. This is one of the screws that holds the side of the headliner to the roof. Same as the bolts, the only way to get to it is through the fabric.