I know more about the subject than I care to. Run don’t walk away. Mold testing is an imperfect science since we cannot even identify all the molds that exist, therefore we cannot test for all of them.
mold can become mycotoxins and cancer causing – look it up. mycotoxins cannot be destroyed as one doctor told me.
nobody wants to give up on a house and move. sometimes the environmental cure is the only cure.
If you cannot breathe due to mold its probably aspergillus which can get inside your lungs and form a fungal ball.
the musty smell is a warning.
your LL will let you out of your lease most likely because he cannot fix it, not because he does not want to fix it.