I will address your challenge, and the answer is that the bank has the collateral evaluated for its own benefit, not for the benefit of the borrower and owes no fiduciary duty to the borrower regarding the evaluation of the collateral.
You are really confusing this situation with the situation where the bank is doing something on behalf of the customer, such as taking and holding the customer's deposits for safe keeping or where the bank is appointed as the trustee of a trust account, etc. In such cases, their is a fiduciary duty that clearly arises. Contrast this to the case where a customer takes out a mortgage loan from the bank. The bank is not making this loan on behalf of the customer or the customer's interests, but is, in essence, simply selling money to the customer. Clearly, the fact that the bank lends money to a customer does not establish a fiduciary realtionship, otherwise the bank would be unable to charge interest on the loan. The bank evaluates the collateral not to protect the customer or the customer's interests, but to protect the bank's interests. Thus, there is no fiduciary relationship established based upon the bank merely evaluating the collateral. In fact, regarding the collateral on a mortgage loan, it is much more arguable that any fiduciary relationship that exists would be the fiduciary relationship of the borrower to maintain the collaterall and keep it properly insured per the borrower's contractual obligations.
The Duckman is correct in that you have a complete misunderstanding of the concept of fiduciary duty as it applies in this case.