This. I'd go one step further and completely purge the idea of the Phoenix Force having any other "host," but Jean. Not past. Not present. Not future. No, not even Rachel. And then let's deal with the Dark Phoenix aspect of Jean's nature head on without any ridiculous retcons. Jean was Phoenix. Then she was corrupted by Mastermind with devices provided by the White Queen, before she had matured into her true mantle (which is when she was most vulnerable), thus becoming Dark Phoenix. And because of their tampering, she killed a billion souls and chose to kill that incarnation of herself in order to prevent far greater atrocities. Pick up the story from there. The struggle for redemption.