I am working on procedures. Would you consider this a way to calculate how much to refund a servicemember retroactively to active duty date?

Beginning with the date they went on active duty, interest in excess of 6% and any late charges assessed must be forgiven and refunded even if already paid. For example, assuming today is May 1, 2018, if their loan was made January 1, 2017, and they went on active duty July 1, 2017, then any interest in excess of 6% and any late charges paid between July 1, 2017 and today May 1, 2018 must be refunded.

To determine how much interest to refund, you could take the difference between the old payment and the new re-amortized payment and multiply it by the number of payments between the time they went on active duty and today. Assume the old payment was $155 and the new payment was $150 in the example above. If there were 11 payments between July 1, 2017 (active duty date) and today May 1, 2018, $55 would be refunded to the borrower ($155 - $150 = $5 X 11 payments.) Don’t forget to refund late charges for this same period of time.

If the loan is really simple interest I'd reamortize based on the actual payment history and what was charged and refund the real amount. That removes all doubt.

And when you say "interest plus late fees" I just want to clarify that for SCRA, late fees are interest. I see being specific to ensure it is included, but the wording just bothers me a bit.