Not sure that it made any sence to mee
I?ve never heard of using roman numerals but that might just be me
As to return to the C13 vs. C7add6; i asked my piano teacher who said that he had never heard of a C7add6 beacuse it would be the same thing as a C13. The only reason that he could see for using a C7add6 is if you specificly wan?t to leave the 9th out. He also said that you could take the 9th out of a C13 if desired just as it is possible to remove the 5th to remain the same complexity throughout the melody. Futhermore he told me that it didn?t make any difference if the 13th was an octave above or not. It would still be a 13-cord. It could actually be below as you can transform a cord anyway you like just it?s the same tones. Often you play the cords in the right hand without the tonica (example C in a C7 cord) bacause you play it in the bas with your left hand. Even somtimes you can play a 4/4 bas go-around (or whatever it?s called) divided into 1/4 notes with the tonica as startpoint and just play the cord stacato one 1/8 note after the tonica. It gives the song more life compared to just playing the cord and bas together...