To answer your first question: If i understand correctly in a list view of empfehlungs, you need to display possibly multiple Quelles per empfehlung. This can be done by putting a list view of sources inside the list view for Empfehlung. This looks somewhat like this:
The second question is more complicated. Excel is in the basis not very well equipped in handling many-to-may associations. There are several possible ways, and there are probably more than the ones that came to my mind.
- import one excel sheet per entity and, after initial import into mendix, loop thorugh the imported objects and create the associations for each object using a microflow.
- import one excel sheet containing comma separated values in one of the columns and, after import, create the new objects using a microflow.
- Using xml or json to import the data instead of excel
- create a data grid per entity and export separately
- use a calculated attribute that creates a comma separated list of values, use it in the data grid and export. (be cautious of possible performance impact)
- use the excel exporter marketplace module (please note: increased complexity)
- export to json or xml
I hope this provides some helpful starting points.