Should the remaining debt be repaid to the university's old owner or new owner after more than 15 years of non-payment?

Your conscience is burdened by the installment you haven't paid, and your doubt about the university's accreditation or your disinterest in its degree does not nullify the right; therefore, the amount must be paid.

It is necessary to investigate who is entitled to it—is it the former owner or the current university? And pay it to them.

If it is impossible to reach the rightful claimant, then you should donate that amount as charity on their behalf.