When I remove the amzn-mobi section, the formatting in the resulting mobi file is *way* worse. I realise it's a specific hack for kindlegen, but even in normal calibre processing it does make a difference—I checked by making one of the headings color:red and both Calibre and Kindlegen processing respect that set of rules.
And the *only* situation the resulting paragraphs are centre aligned is in the old mobi output: ePub, new kindles, eInk kindles, everything else formats it properly (i.e. a blockquote with a <p> paragraph indent). I'm at a total loss as to where it gets the centre instruction from, as there is literally not a single relevant style in the CSS anywhere that has a centre style.