Bryke even said that they never descided who he was. Let's just all assume that he was "accidentally" injured fatally during Toph's labor and everyone chooses not to talk about it. That's what I imagine to cope.
Kuvira's purpose was to help others, essentially, but her way of doing it was making herself the dominant provider and leader of everyone. This is a perfect example of that. She wants everyone to look to her as their leader, including the avatar.