Hi, sorry for getting to you so late!
1. The last heart on Nathalie is locked in the current version. It will be the threesome option and that's still in development. And that brings me to your next question...
4. I might start working on the threesomes in January, after the Christmas update rolls out. I'll probably let the voice of the people on Patreon decide who's going to be with who (though there's not exactly a whole lot of options at the moment; I'd bet you it's going to be Nathalie x Chloe).
2. There's two types of passwords: Special cheaty passwords exclusive for patrons, which instantly let you unlock any girl's stats and non-cheaty passwords for unlocking secret content. You can watch the TV in Adrien's room and pay close attention. They are hidden in the text.
3. Unfortunately, Chloe has only the one date and that's where it stands for now. I got stuck working on the Halloween and Christmas events and had to put her on hold. The same as the threesomes, she will get more content in v1.10 in January.
Hope that helps!