She can file charges all she wants, but it won't lead anywhere. All you need to do is tell the investigator (if there even is an investigation) what happened. Also it would help if you had any of the envelopes showing it was sent back.
Don't worry about going to jail. She loaned you the money and gave permission to use the card. That is not fraud nor theft. Long story short, there is no way she can pursue criminal charges. She can try calling the police, and as I said, they may or may not look into it, but if everything is as you have stated here, they will quickly realize what's going on. Keep in mind that they can look back at her and where she's been living and that would only support your side.
She can file a lawsuit to try to get a judgment against you, however I wouldn't worry about that too much either. Keep in mind she would have to pay probably a couple hundred dollars to file the case.
Even if she does, while it is possible that a judge can order full repayment, I wouldn't worry about that too much either. Basically any reasonable judge would look at the fact that you've tried to pay her but she can't seem to provide you with a reliable place to send the money and the fact that she still won't do so. Keep in mind too that you originally had an agreement to pay back when you could, and that's all the judge would probably hold you too, which is what you were doing anyway. In short, it's not going to go well for her.
Let her talk all she wants. Keep saving up the money that you owe so that you can pay her back at some point, but in the mean time let her go fly a kite. If she really wants her money that badly then she needs to give you a reliable way to get it to her. One suggestion is you might try sending her the money via Western Union or some other similar way. But above all stop worrying about legal troubles with this. She's has been standing in the way of you trying to live up to your end of the deal.