Hi there!
@armingrudd
Regarding “keep_all_columns_selected” option I’m still checking that and if this option does what it sounds like it would be the solution for this problem. Regarding enforce inclusion and exclusion options in GroupBy node - seems to me that enforce inclusion option is actually in background cause newly added column in table are automatically not grouping columns. Except for this uniqueness use cases I don’t see how Enforce exclusion option makes sense in this node but maybe I do not see well 
@malik
I agree with Armin to try to do in a different way so we don’t need to loop over the columns like this.
Also I have seen another topic where similar is discussed (Column List Loop Start) so please if this is the same issue don’t open multiple topics.
Br,
Ivan