Ok the Fire TV Stick remote is definitely detected as a remote (not a keyboard) in Kodi in OSMC:
10:14:25 1630.323608 T:1957511168 DEBUG: LIRC: Update - NEW at 621554:67 0 KEY_UP linux-input-layer (KEY_UP)
10:14:25 1630.323975 T:1957511168 DEBUG: OnKey: 166 (0xa6) pressed, action is Up
10:14:26 1630.746216 T:1957511168 DEBUG: LIRC: Update - NEW at 621977:67 0 KEY_UP_UP linux-input-layer (KEY_UP_UP)
10:14:28 1633.378906 T:1957511168 DEBUG: LIRC: Update - NEW at 624610:67 0 KEY_UP linux-input-layer (KEY_UP)
10:14:28 1633.379150 T:1957511168 DEBUG: OnKey: 166 (0xa6) pressed, action is Up
10:14:28 1633.565186 T:1957511168 DEBUG: LIRC: Update - NEW at 624796:67 0 KEY_UP_UP linux-input-layer (KEY_UP_UP)
The first button press is the UP button on the Fire TV Stick remote, the second button press is the UP button on my OSMC (Vero) remote connected via a GPIO IR receiver, and they are identical scan codes, so whatever works for the osmc remote and other IR remotes (in terms of key mappings) should work for the Fire TV stick remote too. I’ll see if I can work out what’s missing.
This is independent of the commit to add Fire TV stick remote support though - the key mappings there should be fine, customising the actions of the key mappings in a remote.xml / keyboard.xml is a separate issue.