I am fascinated by your absolute rejection of "atonal". I agree that it is kind of a "catch-all" term. Yet so is "classical", for that matter.
As an American English speaker (not to be confused with British English), and in possession of a Music Ed degree, I have never heard of "thesis" and "arsis" as musical terms.
We all know that English is not a pure language, by any means. It is made up of so many different languages that we can't treat it like your Greek. Nor should we. So many words have, as you say, the opposite meaning now than they did a few short hundred years ago. To which I say "so what". That doesn't mean that I don't take language seriously. But I don't get to decide what words mean.
Then there are words that even the dictionary can't define. While performing in a Shakespeare production, I looked up the word "anon".
1. now, straight away.
2. soon, presently.
3. later, sometime.
Or it was an inside joke. The character was saying " Yes, I will be there.....anon" What he meant was "Yeah, right. see ya".
Just some thoughts.