Yes, indeed, it is all those little things.
As for the list, I've already started anyway, see here. Let's see about your other questions for now.
1) It may be that the error is simply because you spelled item in the unit column with a capital i...Do try it out and see if that does the trick. It is always best to use the spelling of the symbol columns of the unit list. If not, let us know.
2) Yes, you can use multiple material codes in one spreadsheet.
3) Segments: Have a look at the example data here, to see how segments were used for different vehicle types.
4) NL data: It is already good that you have a feeling for where/which scale you have data. Next week, you will learn how to do the data gap analysis and that will probably confirm this feeling that you don't have local data. As you know, we will then look to higher scales (NUTS, country) to fill those gaps. It could very well be that you then need to process national data. Important to know is that we do have Eurostat datasets, BUT they have not been processed yet! Therefore, they are not in the system. You will likely have to extract them. We will get there in the next 2 modules: data gaps+downscaling.