I give it up. Thanks to everybody for the time spent...!

I think there is a problem with kernel version 4.5.0-4.1 that did not exist with 4.5.0-3.1.

After all my tests, installs, de-installs and re-installs I had no GUI-access anymore with any of the installed kernels. So once more I did a new clean, updated install from the beginning.

Then I installed the 4.5 kernel, before I tried anything nvidia/prime and before I added more repos (packman etc.). But 4.5.0-4.1 kernel did not boot to GUI (KDE) as 4.5.0-3.1 did. Instead, at the console login, I had several nouveau error messages (sorry I don't remember them). shutting down did not finish, it was stuck somewhere and I had to reset the cruel way...

So, kernel 4.5 (with it's working touchpad) seems to not like me, or at least not my machine :-) and I stay with his (or her) little brother or sister 4.1. So at least I have a standard system and with eventual future problems I can search for standard solutions on leap...

I am now beginning to install all my personal stuff, programs etc., which is a lot of work, so I will not use this machine anymore for risky tests.

Once more: thanks to the community for it's help!
Daniel