"the accompaniment Bellamy put to songs like Death is not the end..."

Thanks for posting that, Phil. You've found an arrangement that the OP could play on his existing instrument - in fact I don't think PB's fingers ever move from the home row. So, OK, you could play some (though not of all) of PB's accompaniments on 20 buttons.

Fan of Peter though I always was, that accompaniment demonstrates to me exactly the limitations of sticking to the home row. Assuming for the moment that you're in C (the recording seems to be in D so is presumably played on a D box, and who knows how many buttons it had), all the home buttons (CGCEG) give versions of a C chord on the push, whereas on the pull you have GBDFA. So the bottom three buttons on the pull give you a full and plummy G chord, the top two a lightweight F chord, the top three a Dm, and other combinations create varying degrees of unpleasantness. On 'Death' you can hear how thin the IV chord sounds compared to the V.

If I were accompanying that song, I'd not play the melody, I'd get rid of a lot of the bellows movement and chord changes (possibly using duplicated notes as Richard Mellish described early in the thread), and maybe try out an alternative chord or two. But that's what I would do. What Peter did was what Peter did (though 'Death' is one of his less sophisticated accompaniments) and, as Howard said, it shouldn't have worked but it did.