Yea, leave some ground around the tombstone so that they don't spawn on the other side of the moat.
They have to be really low in order to trigger it. In all of my time playing, I play generational challenges and I've had countless Sims die on the lot. No more than 2-3 haunt on any one night. I've had a large number of Sims be scared by their relatives. Only 1 has died and her husband was home and was able to plead for her life and won.
The only other time I came close to losing a Sim was when night a Sim got scared like 3 times in short succession. And that was only one time ever.
Knowledge Sims will actually sometimes want to be scared by ghosts.
It's perfectly fine to move the location of a grave/urn on a lot.
You can move Sims just fine. Just remember that you can't bulldoze a lot or place it into the lot bin with a grave on it unless you have the mod that prevents the game from messing with the Sim's files when their grave is deleted.
Sims can be moved freely between a neighborhood and any attached sub neighborhoods. Just move them to the Sim Bin first. They will not lose anything except the value of things that don't get sold when you sell a house.
