Finally yesterday evening the crown test was successfully completed:
However I was not able to home the axis with the dual end stops, move the pen to the new origin of the paper sheet and reset the origin to that new position.
I thought that should be possible?!
That should be possible. Where did it not work?
I homed X and Y and then moved the pen the its new origin on my sheet of paper via Repetier move buttons and thought I can somehow reset that position to be 0,0,0.
But I did not succeed as both Repetier and my LCD still showed the old values for the respective axes.
What is the trick? 
G92 X0 Y0 Z0 will set it in marlin @isathome in repetier will set it in RH.
I tried that in the G-Code edit field in RH.
So best is to define one button with homeing of x and y and another button with resetting the origin?
Will verify that tomorrow I think or maybe hereafter.
RH keeps its own record of coordinates, and doesn’t trust Marlin for some reason. So you also need to send isathome. Octoprint or other gcode senders don’t have that problem.
Jeff you are my hero! 
I programmed the two buttons 1 and 2 in RH with homing X and Y and with resetting the origin to 0,0,0 on the new position.
However RH rather wanted
G28 X
G28 Y
than
G28 X Y
And I used
G92 X0 Y0 Z0
@isathome
for the resetting origin button.
Before I could verify your suggestion I had to print my slightly modified end stop trigger parts and a gauge to position those properly:
After all this my MPCNC properly homed and I could move the pen to the new starting point, reset the origin and could draw something with the still squared machine. 
Now I can draw something beautiful for my girlfriend so that she is appreciating that hobby as well. 
When I have time I have to check the squareness of my machine without homing as this is quite off…
But I love this community and all the help on my way! Thank you!
