Hi,
I have 3 RPi3B's, all with IR receiver connected to GPIO pins, and old HP Media center remote (RC6 protocol as MCE remote).
I'm using some buttons remapped, the "Power" to send the specific "Power" IR code for my TV (GPIO IR Diode), "Pause" remapped to "Stop", and the "subtitle" remapped to "PlayerProcessInfo" in case of playing media and to "ContextMenu" for anything else. And all is done using this remote.xml file in userdata\keymaps:
<?xml version="1.0" encoding="UTF-8"?>
<keymap>
<global>
<remote>
<pause>Stop</pause>
<power>System.Exec("/storage/.kodi/userdata/TV_Pwr.sh")</power>
<subtitle>ContextMenu</subtitle>
</remote>
</global>
<FullscreenVideo>
<remote>
<subtitle>PlayerProcessInfo</subtitle>
</remote>
</FullscreenVideo>
</keymap>
The "TV_Pwr.sh" is for sending to Kodi a player-stop command (kodi-send --action="Stop") and send the IR code for TV (ir-ctl -S nec:0x408 -S nec:0x408 -S nec:0x408 -S nec:0x408 -S nec:0x408 for my LG).
If your remote codes is recognized (as standard protocol: nec), did you tried to use only your remote.xml (...<one>seek(-15)</one>...) without anything else?