[Karellen]

where are all the png files mentioned in the xml files such as osddialogdisplay.png or osdpause.png

In \addons\skin.xonfluence\media\ and it is the textures.xbt file that contains all the flags and icons.
You need to unpack it using Texture Tool... https://kodi.wiki/view/TextureTool

Can't help you with the other part of your post.