So the ones who finally pass on peacefully can either go away in their own or be taken where they need to go by a friendly spirit...while others who roam to much around get hunted down and eaten by others if they don't find a way to move on...
To me it seems like there's a point of no (or rare) return? Like if a spirit has grievances and obsessions and whatnot it starts to pick up some stank. Over time it's form, speech, and behavior get more and more distorted and it maybe starts consuming other ghosts. At that point it probably has to be forcibly exorcised or eaten.
So only spirits who were relatively at peace when they died can pass on by themselves or with the help of a friendly spirit*, either immediately or after completing a clear, pure goal**. They can also retain their form and identity, and might also have some degree of immunity against other spirits?
*Maybe spirits like the wheelchair spirit are for those who want to pass on in a group?
**Probably possible for a spirit that was doing fine to go bad if it's goal can't be completed in a timely manner or if something goes wrong. Like the spirit who wanted to tell his wife the combination to the safe probably went too long. Maybe the wife and daughter were able to last longer because they had each other?