Is it possible to edit the edge lines in a way, that 2d mapping is possible outside the 1D-domain?
Every time I edit the edge lines and then compute the Stored map in RAAS mapper, the old lines at the borders of the 1D-domain re-appear ...
I edit them in RAS-mapper, save the edits - but it won't help :(
The image you presented from the post is a 1D/2D model so the flow in the overbank is 2D.
There is not an easy way to extend the wse outside of the cross-sections. For your particular model, is appears the main channel does not contain the flow, so either extend your cross-sections, make the overbank 2D, or add a split reach.
As for near the tributary, you would need to add lateral structures so flow can spill from one cross-section to the next.