Hello Heinz,
Although I am not the BI Expert, trying to answer your query based on whatever little knowledge I have gained while on working on APO Projects.
1. I understand you added two new key figures in your Planning Area. Your Datasource is created based on the same Planning Area. This could be reason it got adapted automatically without the need of any other manual action.
2. Infocube is the Data Target in this case. It requires manual modification to add the key figures and any required fields/characteristics, I think you have already added the keyfigures and but not the fields/characteristics. So these new fields are still missing in infocube.
3. Yes, you need to modify the Data Transformation to do correct mapping between Datasource and Infocube, and then activate it. In order to see NEW fields in Data Transformation you need to have those fields present in Infocube. Please check on this point.
Hope this will give you some clue.
For this kind of questions, I think below is right place to raise the query to get quick answers.
https://scn.sap.com/community/data-warehousing
Thank you
Satish Waghmare