Not sure I understood it properly but maybe you are looking for Unpivoting node to bring multiple column values into one column? Then if you would like to keep only unique vales you can use Duplicate Row Filter node. Or do you need something else?
I’m not sure I understand either. It’s not clearly explain what is it that you want.
Do you mean to get a list of all possible letters that are in the table? So looking into all cells (all rows and all columns), and then return all these letters as a unique list in 1 column?
If that’s the case, then I would with what @ipazin has suggested.
I can’t think of what else do you mean from what you said and from the sample result you generated.