So, after discovering the way Lilypond works on its end-of-line key-cancellations, had some time and created bass line and left-hand piano for lesson 13.
Attached are the score and a draft mp3 (version 1)
Further, I remade left hand piano, created a strumming guitar pattern, so now the score looks and sounds much more logical (version 2)
Further changed the left hand piano, and took out some notes from the 2nd voice, to make it clearer...(version3)
Also, the parts are ready now: