Again I’ve created a much simpler situation involving three boats:
But this time they are all overlapped. The leeward mark has to be left to port.
In position 2 Red reaches the zone. From that moment Red has to give Purple AND Grey markroom.
Purple however has not reached the zone. And does not have to give markroom to Grey as long as she does not enter the zone. Because for her series score it is beneficial that Grey does not finish in the first half of the fleet, Purple decides to luff and force Grey up. She complies with 16.1 and gives enough room to Grey to keep clear.
In my opinion she is allowed to do this.
If not, this chain would never end. Add a fourth or a fifth boat and they would not be allowed to luff as soon as one boat is in the zone, if - as the argument in our seven boat scenario is commented upon - rule 18 was switched on, between all boats.
Rule 18 is on between Red and the other two, but not (yet) between Purple and Grey.