Coupons and Discounts - These SHOULD show up in the cart during the ordering process, otherwise they are being entered incorrectly or there is some other issue that is preventing them from being executed properly. If your customer brings this to your attention prior to an order being placed, tell them NOT to proceed until the credit is being properly applied in the cart during the ordering process. (Then contact Support to assist you in diagnosing the issue.)
The only time we have encountered issues where a site's gallery "loses" security, is when a user attempts to incorrectly forward their web address (URL) using an "I-frame" or when someone frames their gallery site within another page on their separate web content site.
This is why we ONLY recommend domain-mapping and strongly suggest the use of "framing" not be done.