I saw this thread and it works. Figured prolly a buncha people in r/Android TV that were looking for a way to do it. Here's the thread: https://www.reddit.com/r/ShieldAndroidTV/comments/5q61fh/amazon_video_with_fire_tv_remote_on_the_nvidia/
Here's what I did: Install the multilanguage remap apk on STV. Then, go into STV settings and change keyboard from leanback to MKR. Then, click add remap and click the button you want to change, then scroll through the options list and I changed it to enter. Working great. The op was correct though that if you exit amazon all the way, it will deactivate it. Don't exit just press home button. Worst case, if you exit and it deactivates remap, double press home button, and click the x to remove amazon from the recent list. That immediately reactivated it for me. Good thing is it survived a reboot too. Thanks to op /u/ASiDiE for the fix. Edit: I realize those instructions aren't too clear lol. Let me know if anyone has a question. Was very easy to do.
This works.
I even remapped the FF and RW buttons to control system volume. This is great.
Edit: Oh yeah, one other function that never worked for me on the Fire TV remote is long-press of the back button to open the sleep/restart menu. This works fine on the Shield remote, but holding down the back button on the fire TV remote doesn't do anything.
Wonder if this app can fix that as well.