Here is what I believed happened to these characters that were not in the church in the FS aka Purgatory.
Walt took the role as island protector from Hurley. They always had a special bond, especially when he visited Hurley at the insane asylum and told him Locke came to see him. He was not dead, he was the Jacob 4.0 and protector therefore he did not need to crossover. When he dies, he will have to experience this with most likely a different cast of characters.
Michael was a lost soul on the island and can never leave. I would not be surprised is the MIB is a lost soul and a whisper on the island now too.
Aaron died...sometime, somewhere. It could have been 90 years after Jack's death and the island episode last night for a all we know. There is no birth in purgatory, so he had to be dead.
Ben stayed behind to get Alex and Danielle. He wants to crossover with them, possibly his father and see that everyone else who was ever on the island leaves the FS world too.