Can a bank avoid liability if a bank executive commits a scam in the bank’s name?

If a bank executive (read: employee) commits a scam, then the burden of proof lies on the bank and the investigating LEA to establish just that. If the account owner had no part in the scam and were genuinely scammed by an employee of the bank, by their actions, then the bank is responsible.

This is why banks have Errors & Omissions insurance, in addition to their own risk & compliance assigning and setting aside money for such unforeseen events in which the bank may be liable.