A.
Crossreference:
https://links.jstor.org/sici?sici=0027-4666(199603)137%3A1837%3C14%3AMM%26C%3E2.0.CO%3B2-MYou'll find that this link (above) says that there's more to this music than the numbers. It even calls L'escalier du Diable "flamboyant", rising "majestically", and has a series of "apocalyptic chords swinging back and forth demonically, like bells pealing above a thunderous orchestra." "Sonic ideas expressed" by the composer on the score itself. This is Ligeti at his most "melodramatic, in music overwhelmingly powerful and thrilling."
https://ccrma-www.stanford.edu/~tkunze/pbl/1999_desordre/ligeti.htmlIt may be no more mathematical than a Bach Fugue, but with Bach, he was tied to his musical aesthetic simultaneously, so one should not listen to ligeti like one would listen to Bach.
B.
Not really as obvious. What I mean is that there are people who listen to 20th century and 21st century music and decide that they "must like it" because others do, without understanding the change of aesthetic.
C.Mozart was not so radically different as atonality was. Don't say that. Leading tones and harmonics which bound all of tonality and early modality together no longer applied. It's just a new sphere to explore.
Dan
PS My main point was that she should understand the change in musical aesthetic which i think she missed. It's not about
feeling, and her post suggests that that's what she thought it was about.
PPS I rather like certain pieces of the 20th century. I recommend the Vine Sonata as a first 20th century piece. Messiaen's first vingt regarde would also be good. My philosophy on 20th century is that they are mostly experimental, and sometimes it may not work as well as others, but you can't quite pidgeonhole people.
My personal aesthetic ties me, usually, to the "obsolete" path. Best wishes with your own.