Cecil hit the nail on the head. Even without current Bertia being as kind hearted as she is, Heronia would have only gotten so far in the harem ending as her true personality would have allowed her to- which is to say, not very far. She can say all the right things, trigger all the right events, but at the end of the day this universe doesn't act like a game, but a living, breathing world full of people with their own problems and complexities. Sure, Pii would have definitely been able to use its light magic to make the guys "fall" for Heronia, but how far would that have really gotten her? She'd live in the lap of luxury, spoiled by Cecil and the others, and then the entire kingdom would probably go bankrupt or probably still go to war because she has no bearings of a real leader or responsible ruler.