I got my Series 3 GW back in November. I have had the same issue where all I want to do is play a move off a USB drive. It's never worked.
I finally figured out a workaround today and you guys may also find this useful.
1. Download the "Downloader" app on the fire tv device (just do this on a single one you'll use as the primary)
2. Download the "VLC" app (its the best app I've found for playing movie content in weird locations (e.g. directories) on an amazon device)
3. At my house I'm a nerd and I have a local web server that hosts all of our media content such as .mp4's, .mkv's, etc. You can open up the downloader app, navigate to this web server, and click on a file. This file then downloads to a directory ON that fire tv device.
4. Open up VLC, refresh the movie list, and it'll discover whatever movies you have uploaded. I used .mp4's and it worked fine.
5. You can now either trigger playing these from the front console OR the rear tv. It doesn't matter. In the front console, Media > Rear Seat > view the full screen of the fire device you loaded the movies onto. Navigate to the VLC app, start playing the movie.
6. You can now "Listen" in on this tv, it'll play thru the car speakers. You can also then select the 2nd fire tv, trigger mirroing.
I have the 21D2 firmware right now and confirmed this all worked today.
It's not ideal, but its way better then not having any local content at all. My fire tv's appear to have around 7GB of space on them, so if I use some good compression with Handbrake, I can fit about 6 movies for the kids this way.