**Update: just checked for another update on the firestick since another user mentioned it, sure enough, now it works. Just update again.
I had the same problem and went through all those steps too. What was weird to me was that I ended up removing my firestick and replacing it with an old but still working one I had. I no longer dealt with that “video not available” message, it played my whole movie, no problem. BUT. Suddenly on my screen, I guess because of the automatic updates option on, my screen went black and said it was downloading the latest version of whatever, so after it was done, my Apple TV app was right back to “video not available”. It’s something to do with this latest update. I called Amazon firestick support since apple’s support time wasn’t helpful at all previously to this, let them know what happened. I kinda hoped there was a way to go around this & return to the previous version somehow, but obv not. I asked the rep if Apple TV is now no longer compatible with the newest update, was put on a long hold then they said “Oh, it’s no longer not”. I get the feeling they just went along with me, but if everyone else’s reps are looking into the issue then mine was a moron. Hope to hear good news soon and all returns back to normal.