I had been just willing to take the loss, get evicted, find a new place, and have that on my credit

Generally with you get evicted you cannot find another landlord willing to rent to you.

She’d been denied the one place she applied for because she had on her rents record that she was taken to housing court

Yeah, that'll happen.

What could happen if she doesn’t move out?

Depends on what the lease says but it might convert into a month-to-month tenancy, or renew for another year, or she might be a holdover tenant.

Am I liable since my name is on the lease? Could this end up costing me a lot of money, or legal trouble, even though I’ve already moved out?

Potentially yes to all. What does the lease say about the end of the tenancy? Obviously you can't surrender the property fully with a person there.

I rarely recommend this but because you already have a relationship of sorts with Amy's dad, you might want to reach out to him. And have you spoken to the landlord about the situation, have you surrendered your keys, have you scheduled a walkthrough, did you give proper written notice you were vacating?