I cant see the notice thst you got, but not all aircraft are equipped with the same amenities, and a round trip ticket doesn't mean you'll be on the exact same aircraft in both directions. Re. seat back entertainment screens.... Did you original booking confirmation show that this would be available in both directions? If so, and it wasn't then sure, contact Delta. But you catch more flies with honey so I'd suggest toning down your outrage and indignation. They may have needed to do an aircraft swap so a simple, polite inquiry may get you a few thousand sky miles for the inconvenience. If the original confirmation did NOT show in-flight entertainment in both directions, then you have no complaint.

And no, there is no way in hell you deserve to get half your ticket refunded, as Delta did get you where you needed to go....something far more important than whether or not you got to watch a movie along the way. Lastly, just because it's a round trip doesn't mean both directions of travel price out the same. The price can be affected by flight time, date, general supply and demand, etc. so even if a refund was due (it's not) it wouldn't necessarily be half your ticket cost.