You should detect no blade play with normal use in a back lock knife. That means pushing the blade through tough material. A slight amount of up-and-down blade play can be present by grabbing onto the blade and really forcing it up and down. That is normal for a back lock. There should be zero side-to-side blade movement, no matter what design.
According to your post information, your knife is defective. Even though you may never use it and it's only for display, the blade play diminishes the value, both in monetary terms and in pride of ownership. In my opinion, you should return it for that reason. The manufacturer seems to be willing to work with you on this. The other side of the coin is the fact that the knife was a gift as "a keepsake", as you stated. Considering this fact, you need to determine if the intrinsic value of the gift is worth more to you than the monetary and pride values of the knife itself. Tough call when you consider the manufacturer can't guarantee they can repair the knife and may have to replace it.
I'm gonna let you decide this one on your own.