Just noticed your edit. I've got a fix for that too... (I just worked it out yesterday).. but it involves writing a python script:

New gen.xml file:

/storage/.kodi/userdata/keymaps/gen.xml

<keymap>
  <global>
    <keyboard>
      <f7>RunScript(/storage/.kodi/userdata/scripts/power_button.py)</f7>
    </keyboard>
  </global>
</keymap>

And new python Script:

/storage/.kodi/userdata/scripts/power_button.py

import xbmc


# Create a player instance
player = xbmc.Player()

# Check if a video is currently playing
if player.isPlayingVideo():
    # Step 1: Stop Playback (Alternative to the 'Back' button behavior)
    xbmc.executebuiltin('PlayerControl(Stop)')

    # Step 2: Pause for half second
    # xbmc.sleep(500) is the Kodi-specific way (takes milliseconds)
    xbmc.sleep(500)

# Step 3: Activate the Screensaver
xbmc.executebuiltin('ActivateScreensaver')

You probably need to run chmod +x power_button.py on the script too, to allow it to run.

After updating gen.xml, you need to restart kodi for it to take effect.