Thank you for the information and the link, hbberlin. At last!
I find it slightly annoying that cookie crumbler posted several times without giving us the information. How are we supposed to guess it?
(And I'm still not wholly clear: der Student/Schüler, von dem MR berichtet #8 - do you mean that though he writes in the first person he's taking on a different, more generalised - and more social-media-dependent - persona? And I still don't know whether he's deliberately adapting his style in order to represent this, or whether this is his normal style. In other words, is the text slightly parodistic, or not?)
I don't see why we should all have to research this ourselves. I did in fact try to google 'M Rosenbaum', but didn't get far with just the initial and gave up at that point.
About the comma, I do agree that it would be more correct to have it (though, as has been suggested, it might have been deliberately omitted on stylistic grounds). But I find the whole passage slightly hard work, not specifically that bit.
By the way, when I wrote 'one comma more nor less', in the last line of #9, that should of course have been 'more or less'. I think it happened because I was considering writing 'one comma is neither here nor there.'