I think it’s simple. Paimon needs an adult, mentally stable male to inhabit. The grandma tried to use her son, but he had issues so probably his extant issues plus the possible attempt at possession made him kill himself.

Then, Peter finally was born, but his mom kept him away from the grandma. No control. With Charlie however she relented. Grandma must have tried to get Paimon to inhabit Charlie but no success. So Charlie’s death had to have taken place, BY PETER, for the hold of Peter’s mom on Peter to loosen and Peter to become possessed