If they are then they should definitely be forgiven. I mean, it wasn't them doing the deed. True, they may feel terrible and guilty, but IT WASN'T THEM. Now this may seem childish and immature, but superheroes should create a question or code phrase that only the true hero can answer/say to help prove if the hero is possessed, but the possessor may be able to tap into the mind or something. I don't know.... Not too creative with that.
Joker would be good for Spider-man. The Joker could pose a big threat to him and it would be a big and new adventure for him. On the other hand, I feel that the Joker should just stay true to Batman and DC.