You can't let this "landlady" keep doing this, please! I have a feeling you will have trouble there as long as you live there - if you ever get to move in. I would report her ASAP, even if you only have one receipt so far. That's one month you've got proof that you paid and weren't allowed to live there. Plus if there is mold and nothing has been done to REALLY get rid of it, it can affect your health, which I'm sure you know already.
Is there another place you guys could find?
Take pictures of the place - everything that is wrong. Document everything and report her. Don't let her get away with treating you like this. It's just wrong!
I hate to see you going through so much when you have such a hard school year coming up. I hope you guys find another place to live. You deserve better.
Hugs