Thanks.
1)I wouldn't say there is monogamous tradition. First of all we don't know much about the people outside of the Citadel and how they live. As the game progress you can hear a few times that most people just gave in to their dirty desires. That's what Cathy talks about. How no one sees the point in love anymore. Therefore, it can be assumed that everyone lives as they wish. No monogamous. But if you look at our characters, then yes. They are mostly monogamous coz it's the idea. Half of them are from the past, which is normal. The other half are just like that. They are different from other people of the future. And having a harem isn't something I'm planning.
Second, Phoenix actually has a program called "Revival(I'm not sure how I named it in English, it was a long time ago)". Octavia mentions it during your first meet if you decide not to stand up for Cathy. I thought about exploring this later, but decided to bury this idea as I didn't have much time for that plus it would only be appropriate for those who are on the sub path. What it is is actually a mandatory program for all young people. Males should be checked first and then "milked" if their semen meets the requirements. And then all young women must undergo the procedure of impregnating. But given the infertility rate and population, even this program will not save them.
I won't have harem in my game. But there might be some throuple possible endings.
2)Yes, she is a custom character. She uses one main base and plenty of modifications.
3)Yes, Ellie will take part in the story and is a potential love interest. But only under certain circumstances. As for the mother, no. Mom is not a romancable character.