Thanks so much, Alex!
It seems that I didn’t overlook anything and that the described behaviour is a matter of fact (fields filled in later [workaround possible] or added to the questionnaire later [no workaround] will remain “missing”, i.e. not be part of the transformation).
So, the decision whether or not to embark on Power BI as software to make use of the collected data seems to rely on whether I find some Power BI specialist who tells me that the transformation rules for such later added fields can be added in a "surgical intervention’ without having to start the cleaning, modelling and report building from scratch.
