When you do a LS/PR reimbursement on a simple interest loan you must do more than simply reimburse for the sins of the past and reduce future payments. You must also write down (charge-off) a portion of the principal balance, reduce the contract interest rate, or apply some combination of the two. You've already discovered what happens if one of these steps is not taken--the loan no longer pays off on the final payment.
Before using LaserPro (or any other document generator) to produce an amortization schedule, you'll have to back into the IR or LA that combines with your reduced PMT. I'd use Excel to calculate the IR or LA haircut.