[Baertram]

The best attempt to find the code that reverts what you did change after your addon loads is to scan the esoui source code (Check the addon named Esoui source code) for the stuff that you did change.

Checking the folder esoui/ingame should be the best place to start.

Like the control names or the start of the control name, as most suffixes will be added in xml, like:
Parent = mycontrol
A label below parent is named ($parent)label which results in mycontrolLabel as the name.

This way you "understand" how the game changes the stuff and where it does it. Then you are able to find the correct place to react on the changes.
Maybe it's not the "ON_ADDON_LOADED" event but some function that the game calls every here and there.

Oh and if you use the gamepad mode be sure to check the shared files/folders as a base and then the gamepad files/folders in the source.