Is it permissible, from a Sharia perspective, to participate in and enter a hall built over an old cemetery, and how can we reconcile boycotting this hall with accepting invitations to it, knowing that most of the town's weddings are held there?

It is not permissible to exhume graves as long as the deceased have not decayed and some of their bones remain. What this family did is not permissible, because exhuming graves is not allowed except in two cases: necessity, or if the graves have decayed and no flesh or bone remains. Those who did this must repent to Allah and remove what was built on the endowed cemetery. One who refrains from going to that place is excused because it is usurped.