Yes you are right, the format was updated, although the old format should still work for the most part.
It was indeed not possible for an object to be controlled by multiple options.
For documentation, the old page was painful to maintain. What I would like to do instead is have a "sample" tool with a collection of everything possible and many comments in the XML. This tool would be available in the menu as any other so you could experiment with it directly to find the appropriate object and mechanism, and then copy-paste the relevant parts.
For text distance, it is the distance to the center of the label, in pixels. The rectangular area covered by the text is first measured, based on the text itself and font size, and then the center of that rectangle is placed at the textDistance value, along the bisector of the angle.