How can one absolve oneself of liability for a loan that has not been repaid to the bank for 12 years, given that the bank mistakenly granted a discharge, and is it obligatory to donate the value of the loan and its interest, or only the original value?

You must repent to Allah for incurring a usurious loan. This loan remains a debt upon you that is not absolved by the passage of time. It must be repaid to the bank if possible. Charity does not absolve you of it as long as repayment is possible. If you are truly unable to repay it, then you must give it in charity. When repaying the money or giving it in charity, this applies only to the principal of the loan, not to the usurious interest.