So are you saying, you have launched the live ISO as it is and you get not battery icon?

That's weird! Please checksum your download!
Meanwhile I have dug a little into cbatticon source code and could not really find out where the keybinding is located.
Then I tried uninstalling cbatticon and my brightness keys still work

So obviously I was wrong with cbatticon being the provider of this functionality

Also I could not find any other package to be responsible.
Strange also about your solution: When I add a notification with it I do see them when I use my keys - and that tells me that the key identifiers are indeed the same ...

So. This is a bit mysterious to me

Why not just stick with your addition to the keys-file if it works!?