@ThRippJck I wouldn't give up hope just yet. It's my theory that Kenzaburou isn't possessing Grace's body 1 to 1 but in some ways is acting as Grace's brand new conscience and something like a guardian angel to her. Maybe eventually he'll return home and Grace will be left to develop the new start with Anna he helped give her. (would fit the theme of him being an amazing dad, too)
Sorta parallel to the relationship a real life VN player might have to a VN protagonist. We too step into their shoes long enough to guide them to a better place and then take our leave once we get the good ending. Which in this case I'm pretty certain will either be a yuri or harem ending.
I think if we were meant to get a story about Kenzaburou truly
becoming Grace, we wouldn't see his silhouette/appearance all over the place and at very most his old appearance would be in a very occasional flashback.