Your best bet is to set the staff members - who can’t “seem to grasp looking at a date on a receipt” - with the “approval required” condition for refunding and exchanging orders.
So then whenever they go to do a refund or exchange, they need to get a manager/boss (or someone who knows how to read receipts) to enter their pin to approve the transaction, so that they can proceed with the refund.
To answer your question directly – I don’t believe there’s a way to set a return window for POS at this time. I found the same help docs you did, which shows how to set this up for the online store (but it doesn’t mention anything about POS).