Very interesting this topic is...
Though i haven't understood point B). Are you meaning that you can decide not to apply the support modifiers in a combat?, which is the profit?. If these units don't support the combat it supposed that they are not in the combat but... I don't understan "block 1st rank", sorry for my question O_o
In Warmaster supporting units could block retreat; you decide it when the combat has been resolved.
In some situation that could be a good option to sacrifice 1st rank to avoid losing combat and having 2nd rank involved in a combat you would avoid.
Example: 1st rank is composed of Goblins, 2nd and 3rd rank are composed of Orcs. You probably don't want that the 2nd rank of Orcs are part of the combat because you will most probably loose it and if you loose it the pursuer will probably flank you, etc.
In Warmaster Ancient you no longer have this option.
It is not a real issue in Ancient since sacrifiable troops are skirmishers and would most probably dodge the charge by the way.
In Fantasy sacrifiable troops are not skirmishers, so that could be an issue.
Well, most players don't see a real difference, but some do

I do, mainly because I'm playing Orcs a specific way. Some Skaven or Khemri players could do as well.
So we are playtesting this rule to see if we include it in WMMF.