You have listed some very good ones! Footwork to me is a lot of things, Speed, balance, shifting, lateral mobility, side stepping, getting quick angles etc... A few more: Benny Leonard Vasyl Lomachenko ( best footwork in the game today )
I'll add he must have had the ability to throw and land punches with balance while moving quickly. This combined with great stamina to throw punches and a great chin to take them, mixed in with rule bending or fouls made him very difficult to beat! I actually think Greb could be vulnerable to taller and longer boxers with a good jab and outfighting skills, OR a big punching man at 160-175 with an inside power game. Two tough match up's for him. Most of his best opponents did not fit this description. There might have been some Slapsy Maxies or Fritzie Zivic in Greb too. What do you think about this, Chuck?