Hello and welcome, Threesacompany! Our mods will move your post promptly to the New to Polyamory, where it belongs. Also, in further posts, please use more spacing - I kept falling of the line I was supposed to be reading because there was so little visual direction.
From a quick read it seems that your partner's ex seems to enjoy the financial, emotional and sexual benefits of being together with you but is uninterested in holding his end of the bargain. Without you there, he and your partner have fallen into the same pattern that lead their previous relationship to divorce.
Why do you think that these two can't continue to work on their relationship from different addresses? And is it really impossible to secure quality childcare in your area so that your partner can continue to work and study?
Being that it is your partner who is in a relationship with him, it is her house they are living in and it is she who has to face the day-to-day drill of living with him, I can't see really no other way for you than to support her in asking him to leave. The two of you can be friends without him living there, right?
Me: bi female in my twenties