Hi,
I'm Trinidad at your service, developer for more than a decade of professional experience with a vast range or technologies, from desktop, to web, and embedded development.
My proposal is to fulfill your requirements by using C++ with Qt on a stock Raspbian linux OS.
This will allow for graphic UI with user input and a configuration menu, static images and videos, and networking to be implemented. All of this in a portable fashion thanks to the well established Qt libraries, even allowing for flexibility for future expansion, also allowing to port the system with only minor changes to other platforms if needed, and so on.
Using a Raspbian, or an existing Raspbian derived, linux distribution as OS will make easy the system deployment as it will be just a stock distribution with the software installed on top, no hassle.
Using C++ will also help with keeping the code safe. Python code can be easily read and copied, and C# code can be de-compiled easily, but the same is not true for C++. On top of that, an option would be for the system to communicate to a central server to ask for permission to run, so the server could control what instances are allowed based on a system identifier, like the MAC address or IP, for example. This could be compounded with cryptography if needed.
Please feel free to contact me if you have any questions.
Thanks for reading my proposal.