oops - as i mentioned before, i am really a newbie on this.
first of all, thank you for your patience!
I am afraid, my mistakes startet even earlier.
In fact when i set up the child theme i did not name it in the childTheme.php - instead i litterally used the mytheme as name.
however, if i insert the name of the child in the php file, the site does not show anything but errors.
I thought, it worked with just this setup:
after disabling the grav-logo.png import anything behaved, as i expected and I was able to use any template or partial - if a *.html.twig was available in any of the template folders.
with the suggested changes built in , the site does not work at all - so i am afraid, i got most of the explanation on expanding themes wrong. Last not least when i did not gave the right theme name, the jschell.php did nothing at all - and i now wonder why and how this worked … but i will find out ^^
so i will first read the docu yet another time before wasting more of your time.
nevertheless - thank you once again for your time
… i am afraid i will be back very soon for further questions 
best regards
odde