You would have to find a compromise and somewhat expose the main girls a little. Otherwise the board's satisfaction would quickly decay and the game would result in game-over.
There will be, however, some key points in the story where you'll be able to go against the board's wishes and protect the main girls from exposing themselves or doing extreme acts, which would result in an alternate story path. So, yes, you can go against the 'Board and choose another approach'.
As for a 'Harem Final'. I don't want to spoil endings, but yeah, you can romance the Main Girls. Not the Secondary Characters, though. They might still like you and it'll change some events if they do, but it won't be a deep relationship with them. So, if you consider just the main girls a 'harem final', then yeah, sure.