I just flew ATL-SFO on UA. We had seatback screens, but you had to buy the DirectTV or movies. The "free" wifi entertainment was, in a word, awful. A few obscure documentaries, and maybe a few kiddie things. I didn't get DirectTV free on my flight). And I didn't need to download the app. I just logged in from my web browser as per the instructions in the in-flight magazine.
Downloading videos ahead of time is a much better bet.