As you stated, the DA FORM 4856 specifically states:
“This form will be destroyed upon: Reassignment (Other than rehabilitative transfers), separations at ETS, or upon retirement”.
Therefore the only way I believe you could legally keep copies for 1 year would be to have the Commander implement a local policy that states you will retain all DA FORM 4856 for 1 year in the event a student wants to contest his AER. This should provide sufficient justification. I would also run this policy through JAG or the IG and get their approval on the policy.