This is your blog so I'm only going to comment once. (Blogs are not generally supposed to be for feedback or commenting upon by others.)
You and Airyn have been played from my perspective. There are two ways people get away from bad situations. One, they figure out what they need to do to survive and how to get what they need to live on their own. Then they work hard to make that happen. And they leave. They know they can manage their own life. Or, two, they figure out who can help them leave, glom onto them, and trade whatever they've got to get out. These folks don't know how to be independent, they only know how to manipulate, charm and trade to get what they want.
Of course she initially presented as being into you. Not because you are not attractive but because you and your husband were a package deal at the time. To get out she had to be into both of you. She's not a lesbian, she's not even bisexual or biamorous. Did she become not into you once she moved in? I bet she did. She realized that she could stay if she wasn't dating you because she is still seeing your husband. Who has lost his mind and all perspective over a 21 year old body.
She is employing the same tactics as her abusive, manipulative mother. It takes work and self-awareness to not repeat what our parents taught us. She does not give the impression that she is willing to work or particularly self-aware. She is a prime emotional manipulator. She can read you and Airyn like a book. (Manipulation does not require self-awareness beyond a baseline 'I do this so they do that.') That whole "Love ya!" business is another way to get a hook into you. As she doesn't want you sexually, then maybe she can give you a taste of emotional connection and keep you hooked that way. It is no coincidence this happened after your husband's mom gave him an earful about Chipmunk. He may not listen to you but I bet his mom got his attention, at least somewhat. She needs to keep you off-balanced enough so you don't connect the dots and fully realize how destructive she is to you, Airyn, and your marriage. And to herself.
She is terribly needy. WAY WAY more than even your typical self-involved early twenties woman. She is a black hole of need that cannot be filled by your or Airyn or anyone else. Don't excuse this by blaming youth. Yes, of course, she is immature. I was too at that age. You were too. But you cannot help her. Airyn cannot help her. There is something terribly awry with this woman beyond the normal growing pains of young adulthood.
I know you made a promise to her. But you are not actually responsible for her. You may feel responsible but that is not true. It is not real. Airyn is not responsible for her wellbeing either. She is an adult.
You also made a promise to Airyn and a promise to the marriage both of you share. You have invited a toxic cowgirl into your midst. If she does not leave your house, your marriage will implode. Get her out of your house. Give her money for an apartment first and last payment if you have to. (Don't pay her rent!) If Chipmunk and Airyn want to continue their relationship after her moving out, more power to them. (This won't happen-once she doesn't get what she wants from you two, he will be the next to be dumped. Don't believe me? Try it.) Help her move. Wish her well. But get her out.
I'm sorry to be so harsh about someone you like and want to be happy. It's not something I want to write. But it's what I see from the outside. I don't think she is evil incarnate. But she is toxic and will take down your marriage with her if you are not careful.