When I wrote that guide I purposely only detailed (for the most part) where our custom keymap deviated from stock Kodi behavior.
Outlining “expected behavior of all keys” is not quite as straightforward of a question as I think you are imagining it to be. What any particular button does depends on what screen your on. For example if your using Estuary, not playing anything, and you press the play button on an album in the home screen (widget) it does nothing as this is the way Kodi is programed to operate. If you are on the same screen and you are playing something then the play button toggles play and pause. If however you click OK on that album it lists the songs and pressing the play button will start playing the currently selected song. Now that there is a song playing highlighting a new song and hitting the play button will not play this new track as it now is play/pause for the playing music. Hopefully you can see how this would be easier for someone to discover the behavior by playing around with it than to try to understand a bunch of ‘if this, then that’ breakdowns.
With a remote that does not contain dedicated ff/rw and chap +/- buttons you cannot (user customization excepted) control these functions from anywhere other than the full screen music playback window. The reason being is that the only logical keys are the direction buttons and those are needed for navigating the UI. What you can do is when music is playing you hold down (long-press) the “home” button which will switch to where you want to be. When you are in this window the up/down direction buttons will act as next/previous track, left/right direction (shot-press) skipstep, and left/right direction long-press rw/ff.
That being said music control via remote without a display is challenging with this type of remote due to the variability of what the buttons do depending on the current state of the UI. In this situation a Kodi remote app on a phone or tablet would be much easier. Their is a web interface you can use as well, although in practice it is normally not as easy to use as an app. Barring that you could use a remote with more buttons.