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Tested the code - it runs, and produces the output I expected, but I can't be sure if that is what you wanted.
It occurred to me that it is possible that the "conditions" require some sort of...

About var:

I think a good general guideline is: use it if it helps avoid needless repetition.
As in:

var aDictionary = new Dictionary<string, List<DataItems>>();

vs

Dictionary<string,...

Ok, I went through your code and I think I see what you are trying to do, but you are making it more complicated than it needs to be, I think.

In essence, you have a 2D array of inputs which...

If I understood you well, although subgroup IDs (keys) are not unique for elements, they are unique per subgroup, and you need to be able to dynamically change the number of elements for each...