The fact that they zeroed it on the books has nothing to do with the debtor’s obligations. It doesn’t necessarily relieve the debtor of all obligation; it may be merely a bookkeeping entry for the loan company or a regulatory requirement for proper reporting to stockholders/customers.
Rights under a contract (even a loan contract) are generally assignable, provided there is no language to the contrary in the document, or no statutory provision. Perhaps there is a statute limiting such rights in the case of residential mortgages, but I’ve never heard of one.
If the loan company is in bankruptcy, a creditor (or the bankruptcy trustee) might assume the company’s rights under the loan.
It is not a trustee, but someone that claims to have purchased the loan.