Summary of the facts:
5 boats of the Star-class all with port tack and overlapped are sailing downwind towards a leeward rounding mark to be left on port. When the first boat reaches the zone in position 2, the leeward Red boat has no possibility to give room to the four inside overlapped boats.
The Problem is:
In RRS 2009-2012 I found no rule that force Red to sail a course different from the course she was sailing and make it possible to give room early enough before the mark. (The words of the old rules “when they are about to round” are deleted)
Is a boat obligated to give room before the rule requires her to do so?
Well Uli, this is my opinion:
Because of the deletion of "when boats are about to round a mark" the border where rule 18 begins to apply is exactly defined. If one of the boats enters the zone - in your diagram the Blue boat - rule 18 is immediately in force. In this situation Red breaks rule 18.2(b) as soon as Blue enters the zone. She is not giving enough mark-room to all inside boats to sail to the mark.
The only way she can prevent that from happening is to ANTICIPATE!
And act accordingly before one of the boats reaches the zone.
You are correct in stating that there's no rule in the RRS forcing her to sail any other course, but the consequence of sailing it, is that she breaks rule 18.2(b) as soon as one of the boats enters the Zone. If she luffs before any of the boats (including herself) reaches the Zone and stays out, she indeed breaks no rule.
What about 18.2(e) you will ask? That rule states that if a boat is not able to give room she is not required to. Not quite. The exact wording has additional requirements:
(e) If a boat obtained an inside overlap from clear astern and,In this case the relevant requirement is "from the time the overlap began"! According to your diagram there was already an overlap well before the zone, so Red has no 'escape' in that rule.
from the time the overlap began, the outside boat has been
unable to give mark-room, she is not required to give it.