Here's a thought: could the "frame narrative" of the show be set in Heaven? That would be consistent with narration from Dean, and it might be a way to make a memory-wipe scenario palatable. Have a first scene with Dean, John and Mary (the JDM/SS versions) in heaven, where maybe J&M have gotten or are getting some of their memories back. Or, at least, it becomes clear there are some discrepancies in memories, and Dean investigates.
Still not great, necessarily, but at least then there's not the same sense of total waste and frustration, as some version of these characters do get their