Well folks,
It is a way to big task to revert this. System Security needs to take off, even to replace the existing version. But all the effort will not make it. Music 1.5 will not run. Lack of debugging logs are no help. I give up for now.
Maybe some enthusiast will create a manual to accomplish this, I will not.
The only way to get rid of it is to change the Operating System - sounds like a Plan B. ;-)
I took a look to --> http://swinsian.com/ and it looks promising.
There is a 30 days full feature evaluation period before it cost about $40. I will check if it fits to my kind of usage. It is a bit like 'the old days', but this must not be that bad.
Several things, that do not work as expected in Apple Music do work fine in Swinsian, like Cmd-L, the control bar is at the top, a playlist icon does not cover the half of the screen, and so on...
Apple Music must stay for the sync of the iOS devices and other stuff, it can not be uninstalled anyway. I have to check the interoperability.
Maybe Apple should think about a $40 "Pro" Version - this one now with support and usability. I would like to pay for it.