The first part of this file describes the structure of the menus in Krita, it is a very simple but effective method to build them. If you know how to deal with it, you can edit the menus and even add your own entries.
This is a question I can not answer, I’m only a user, not a dev.
To point to your own icons you need Krita’s action files (exactly, create your own for this task). Those are found in the folder actions in your resource folder. Then you will need a tool like the Python Plugin Developer Tools to figure out the ID’s or pointers of the actions you want to give your own icons. I never did this myself, only know roughly about the principle, here you need users with a better knowledge of Krita’s internals.
Michelist