In principle, what stops one party from simply reverse engineering the client app and permanently retrieving the file? Once the video file is decrypted on the client, even briefly, you've lost control.

Your answer demonstrates that you've thought about this problem, not that you've solved it. I like the idea of this app, but I would argue that the core promise of your app is technically infeasible.

EDIT: I clicked through to read your whitepaper and see that you've explicitly called this concern out and admitted the DRM scheme cannot be foolproof. That's admirable, and I'm glad you did address it. I would gently suggest you place that disclaimer somewhere in your FAQ as well.