A major thanks to grimsoul and everyone for their responses.
The tombstones stay on the community lot (I put them next to the chapel) and they do stay there in my case. They can also be moved without going into build mode. I however, haven't visited them yet during the night so I haven't noticed any ghost sightings just yet. But I'm not quite sure if other sims (non-household) can interact, take into inventory or release soul. As of now I've fenced the tombstones off just in case.
Maybe the dynasty challenge can just put precautions on the graveyards in which it only serves as, a graveyard. It's actually really interesting, other than the option of releasing them.