1. I'm not sure yet. Didn't get to the point.
2. Talk to her, give her compliments, go on dates...
3. Make her submissive. Multiple options available. Some with conversation topics, watching porn at home or giving a specific book as a gift. Needs at least 50%. After that you can gift a slave collar. Some characters have special conditions for the collar. Palla for example won't accept it until you complete her story.
4. The dev said that there are multiple endings...