So, I tried for the 4th time calling the MM hotline (while already having the complaint open), this time the main German-speaking line. I explained again to the operator the problem and to my positive shock, the agent actually seemed believably interested on solving the issue. She wrote down all the details of the case, including the exact time of my cancellation phone call and told she will escalate and take care of it and she really did In the afternoon I got a phone call that the cancellation is authorised, although I still do not have the miles refunded, but I at least got an email confirmation that my reservation is cancelled...

So in the end, a happened, but way too stressful (3 previous calls I was just flat-out denied) and I am still scratching my head where the original cancellation disappeared.