Calum, many events use a Whiskey flag to indicate that a boat must return to the starting area. Here is a commonly used instruction:   “If a boat is shown a ‘W’ (Whiskey) flag by an official vessel, it shall return to the starting area. That boat will be scored in its last known position. This changes RRS 28.1, A5.1, and A5.2.”
I recommend avoiding the word “Finish” in your instruction, as it is a definition that may not be changed. 

Well of course it’s the job of the PC to be sure the RoW boat is not pulling a “Hollywood”. Easy to determine when we’re umpiring match racing. All the details are needed in a hearing. 
In Ben’s scenario, “The PC accepted that L's action was made at the last reasonable opportunity and appropriately to avoid collision had W not acted.” Their finding, taken with the previously mentioned cases, supports the result to DSQ W, IMO. 

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.