@WhimsiCat you are correct in the second chapter, it is explained that there is a reverse harem route where Lize is one of the members, but Baldur is also one of the members. While it is the route where both of them live, the reason the route is a no go is because when Kobayashi was explaining the route, when Endo heard it, he thought about the life of the characters after the game was over. If they did the reverse harem route, it wouldn't really be that happy of an ending since, in his words "the crown prince, a marquis daughter, and all sorts of other important young nobemen all fighting over finne!? If you really think about it, if they really start fighting over her, won't it tear the country apart!?"
Thus, after they eliminate the idea that any of the routes, even the solo clear route, should be followed to avoid deaths and the future unhappiness of the characters. And since they realize that they can truly interfere with the game, they decide for the greatest happy ending to allow everyone to be with who they treasure and find their happiness.