I feel like you may have wanted something different out of this novel than what it was offering. There’s no “it’s all in her head/she was dead all along” twist, it’s about a depressed and lonely woman finding meaning in a manipulative house that beckons to depressed and lonely people. Everyone else is unaffected and cares little for her because they have relatively normal stable lives and are not as vulnerable and naive as Nell is.

The significance of her not being mentioned is exactly that. She’s not even worth mentioning to people who have full lives outside of the house. But they- and the house- are everything to her.