Will the deceased be tormented because of an unpaid debt, even after a long period has passed since their death?

Your father's accountability will not be cleared merely by your uncle assuming responsibility for the debt, unless he actually pays it off. This is the view preferred by some scholars, because the soul of a believer remains suspended by his debt until it is paid off for him. In a prophetic tradition, the Prophet, peace and blessings be upon him, did not pray over a deceased person who had a debt. When Abu Qatada assumed responsibility for it, the Prophet, peace and blessings be upon him, prayed over him and said to him: "Now his skin has cooled."