We are on Earth we know today, but with an addition - there is an actual hell, heaven and purgatory. After you die, you go to the purgatory and collect your ticket for the train to either hell or heaven. Let's assume that the guardians of the purgatory decide which world you go to. Your path will be decided through whether you broke any laws (not religious), whether you took care of your body (staying healthy), whether you committed suicide or died of natural causes (committing suicide gets you a ticket to hell), whether you acknowledge your sins etc.
Some people would receive the judgment to come back and finish the tasks they couldn't do before they died. However, they come back as different humans, with only two months to live. They are not allowed to tell anybody about their past self, otherwise their existence will be wiped from the planet's history/timeline. This means that anything they do would be erased from everyone's memories.
Human A is a father of one teenage child. He dies in a car accident but has things he needs to deal with, so he is given the permission to come back as a new person. He is now called Human D. He eventually finds Human B, his wife, and Human C, his child. He befriended them, claiming to be Human A's friend in the past. He manages to survive without telling them who he is - but right at the end of his time, his behaviour and actions get analyzed by B, and this is when B figures out that D is actually A. And so, D has to disappear for breaking the conditions of coming back for two months. D is given a new death cause, while A disappears from everyone's memories. You'd think that if A didn't exist, C cannot exist either. However...
The guardians grant Human A/D the choice to make someone remember him, even if the whole world forgets about him. That someone will have the knowledge of their existence and will keep the memories, but in fact these memories will be considered fictional the second that person is erased from the timeline.
1) Human A chooses Human B to remember him. Because A disappears from the world for good, C doesn't exist anymore. B remembers C and tries to convince everyone about A's and C's existence but fails. In addition, when B was 9, she nearly died in an accident, but she was pulled away by A which saves her. So if A didn't exist, how can I explain that she is still alive? (she would have died in this accident)
2) Human A chooses Human C to remember him. So going back to question 1, A and B don't exist, nor does any relative know who the hell C is. Is there a way to explain C's situation scientifically (no parent DNA)? Or is there a way for C to make everyone remember?