Nice Ang,

You are heading in the direction I was looking for. 

Let's say that the boats had plenty of room for W to do what was needed to keep clear. 

The aim of my scenario was to create a situation where KC actually did comply, but simply, RoW judged it differently, resulting in them both taking action at the same time. 

I would second that for people on a Mac!!

The more I think about this ... the more the reptile-level of my engineering-brain is telling me that the only way that this ILCA gets back to its original position + improved position on the line ... after countering the losses due to wind and water drag forces ... and the initial leeway loss when the sail is initially brought in ... without passing HTW ... is by the input of energy from the body resulting in a fan of the sail. 

Call it just a conservation-of-energy analysis instinct .. I'd have to break it down to each dynamic segment to see if I'm missing something. 

I will return (with trepidation) to my original stance.  In this scenario, unlike on a beam reach for example, I contend that there is only one side that can be chosen by the ROW boat to pass the obstruction and where, therefore, RRS 19 applies.  Blue can chose head up and go behind Yellow.  In this instance, I think we all agree that the obligation is on Green (RRS 19.2b) to give Blue room to pass between her and the obstruction. Blue's obligation (RRS 19.2a) is to give Green room to keep clear as she changes course.
If, however, Blue choses to avoid the obstruction by gybing, she is not passing the obstruction .... indeed, she is not able to pass the obstruction on her port side unless at position 1 she suddenly had a burst of speed and could cross Yellow.  Given that she is unable to pass the obstruction on her port side, she chooses, instead of heading up to pass on her starboard side, to avoid the obstruction by gybing.  Therefore in this instance, RRS 19 does not apply.  Returning to my original post, I believe this is why the language of 'pass' is not used in RRS 20.  

In RRS 20 (sailing close hauled or above), there are things that have to happen for the ROW boat (leeward) to be able to safely maneuver to avoid the obstruction (again, it doesn't say pass).  Should she chose to avoid the obstruction by tacking (rather than passing it astern per RRS 19) she has to follow RRS 20.1 and things unfold from there.

In our scenario, it seems to me that if the intent was that Blue had obligation to give room for Green to gybe between Blue and Yellow (obstruction), there would be another rule entitled ROOM TO GYBE AT AN OBSTRUCTION with a series of hails and responses.  This would be problematic however, for all the reasons that I stated in previous post (broken overlap; uncertain duration of right to room, etc.)  However it seems to me that this is NOT the intent since the ROW boat  going downwind already has the ability to execute the choice to avoid, rather than pass (which they don't going upwind without the protection of RRS 20), and the give-way boat already has the ability to pass the obstruction safely by heading up astern of the obstruction.  

 

I think the approach I've seen used is to tell them they have been finished on the previous lap. (Legally or illegally)

I still think if we see the SIs it will clarify the situation with a phrase like 'The Race Officer may decide to finish boats at any time at his discretion. This may not be the lead boat, but results will be calculated on average lap time to ensure fairness, the aim being to keep the finishing window as short  as practicable' (I selected that frome Penzance Sailing Club who used it for handicap and non-handicap fleets) or from my club 'For a committee boat finish, flag S will be displayed on the committee boat accompanied by two sound signals. For a shore-based finish, two sound signals will be made. Boats will finish the race when they next pass through the start / finish line. Finishing position shall be determined on an average lap basis.'

(Clearly purists will say start/finish line shouldn't be mentioned - but for an average club sailor that's clear and easier to understand than describing a gate. And for the even more purist, a Committee Boat should be called a signal vessel but I can say with certainty the club sailors know what a committee boat is and will think I've lost the plot if I say signal vessel.)