The main girls won't cheat behind MC's back, as in without them knowing that he knows, and knowing that he allows it/encourages it.
The assistant and Emiko are currently major characters, and will have scenes eventually, probably NTR scenes as well.
The pink heart means how much a girl loves the MC, and the purple heart means how much the girl is willing to do. They can both be high without any problems, or just one being high and the other low, w/e. Usually, if you want NTR scenes, the purple heart will be more important, but that's not always the case, it'll all depend on the scene's context.
If you disable sharing, there will be no NTR scenes and even some choices will be hidden, as well as some dialogue changes. I do plan on eventually adding an option to enable/disable sharing midway through a game and not have to rely on that particular choice.