Is the deceased father held accountable for the installment debt if the uncle undertakes to pay it, or not? And should the heirs hasten to pay off the interest-bearing bank loan, or should they continue to pay its scheduled installments, and is it permissible to pay an additional amount to the bank to end the loan in one lump sum?

We advise you to promptly pay off all your father's debts, whether commercial debts or the bank loan. Your uncle's undertaking to pay them off is commendable, and he is absolutely obligated to fulfill this according to the preponderant opinion, as the majority of scholars hold that guaranteeing the debt of an insolvent deceased person is valid and binding. It is preferable to expedite the payment, even if it was deferred, based on the hadith (definition): "Now his skin has become cool." The opinion of the majority of scholars is that deferred debts become due upon the death of the debtor. However, if expediting the payment results in an increase from the bank, you are not obligated to expedite it, and there is no harm in paying at the agreed-upon due date.