I'm not a judge so I'll defer to others more knowledgeable than I. This does, however, look like an interesting case; thanks for posting it.

If I were on a jury, here are some things that I would be looking closely at:

Looking closely at Yellow in position 2, they seem to be taking way more room than needed, even if Green has an outboard motor, since Yellow's bow is pointing 45 degrees from green's path. 

And looking at Yellow in position 3, they seem to have taken a very long time to head up after the duck. 

Given the light air, flat seas, and fairly small keelboats, it seems like Yellow could have taken much less room.

So, this diagram does not help Yellow's case. Is the diagram accurate, or would you want to revise it?

I give you my idea

Al, I agree about Pos 3, but that would not be an unusual track for a "panic" evasive course.

Seems to me there is a lot of open water to starboard for Yellow.  I’d want to understand when it became clear to Yellow that Green was not going to keep clear. 

Given Green’s slow speed, even if she started tacking away at #1, I’m not sure she could keep clear.  If that was also clear to Yellow at #1, she could have turned to port earlier and kept clear of Blue by ducking Blue’s stern or by tacking to starboard. 

In other words, I don’t have enough info to exonerate Yellow for fouling Blue under 43.1(a) for breaking rule 11 using the “compelled” standard. 

Yellow is not ROW v Blue and Blue does not owe Yellow room (Green is not an obstruction) or mark-room (Blue entered the zone clear ahead) so the only possible exoneration avail to Yellow is 43.1(a). 

Another question is how long has Yellow been on this course?  Has she been on the stb layline for a long time or did she just complete a tack from port to stb immediately prior to #1? (to determine if 15 or 16 are in play). 

Here's my take.

From the diagram, it seems that the Green Boat entered the Zone first, followed by Blue and then Yellow. No overlaps existed at the time of entering the Zone. 

Before entering the Zone, Green wouldn't have right of way but, once entering the Zone, was following a proper course rounding of the mark.

Blue then followed a course astern of Green. Yellow 'came in hot' and only formed an overlap once in the Zone. Yellow should have kept astern of Blue and Green and/or, to avoid a collision, emptied her sails until Green rounded the mark. 

That's my take. Yellow was in the wrong.

The OP asks about R 19 but as Green is 'Keep Clear', she is not an obstruction, so R 19 does not apply.
My take is that Green on Port fails to keep clear of Yellow as Yellow has to alter course to avoid her, so Green breaks R 10. R 18 does not apply between them as they are on opposite tacks on a beat to windward R 18.1(a).
 
Yellow enters the zone astern of Blue and has to keep clear R 12 and also give Blue mark room R 18.2(b) and (c).  Yellow becomes overlapped and to weather of Blue and has to keep clear under R 11. Blue alters course to avoid Yellow so Yellow breaks R 11, but is exonerated as she was avoiding Green. 

John

I continue to have problems with the diagram. The original post indicates that all the boats are 26 feet, implying that they're the same class. But when you examine the diagram, Yellow is going about 2x the speed of Blue, and about 4x the speed of Green. Is the diagram accurate?

Her I have a picture taken from Blue of incident; Yellow had been on starboard fore some time before that incident because there was more wind in the starboard side of the leg so she was on more speed.

Looking at these pictures (as a tactician on Yellow) and to my knowledge:  I would have made the following choice, likely avoiding all mishaps - Light winds= keep your speed and there is enough room between green and the mark to tack to port and fetch the mark and avoid collision as ROW boat. I would have hauled that to Green - "Keep clear - no rights!". Green would have to clear some room maybe, but she was not ROW and has to take evasive action. I think Yellow made a bad choice by bearing downwind knowing that Blue was there as well, meaning you go into a cluster head-on. By not bearing down and glide into a tack Yellow also preserves her speed and makes shortest course. Even even Yellow would have hit the mark, or missed it on the other side - she still could continue the race but had to protest against green in order to justify the missing of the mark ( when hitting/missing a mark as ROW boat by avoiding collission you can continue! but have to place a protest)

The rest the way it went down is a "cluster-F" particularly with low speeds - Green was already "dead in the water" and could hardly go anywhere, except slamm her rudder pushing her nose up...  When Yellow avoids a collission with Green by bearing down, Blue also has to step in to avoid Yellow's collission with Green .... how much room was needed there is always disputable ...  But still, tactically Yellow should have made another decision and tack and glide for the mark.

What is the option for Yellow?

The Blue is a ROW boat vs Yellow, so she has no right for room and is passing that gap between Blue and Green except on her own risk.

She could have duck behind Green and Blue and lost lot of boat length

Or she hauled to Green - "Keep clear - no rights! Tack to port, Fetcing the mark to port and hitting/missing the mark, protest and continue in the protest room...

To misquote from a film, green is a very naughty boy.  Breaks rule 10.  From the description, no possibility of exoneration from 15 or 16.  Green DSQ.  Yellow may have had other options for avoiding collision but when it became clear that green was taking no avoiding action, I think yellow's course was reasonable.  

Between 2 and 3, yellow is not keeping clear of blue, she breaks 11 and is exonerated under 43.1.  At 3, yellow luffs to tack and there is contact with blue.  At this point, yellow (give way boat with respect to blue) breaks 14 and is not exonerated.  With her greater speed, yellow could have avoided contact without luffing to tack.  Yellow DSQ.

Interesting situation.
Was Yellow compelled to break rule 11? I do not think so as contact was made when she alter course to tack. Blue was ROW boat before the Yellow entered the zone. So, I have an issue why Yellow would be exonerated for room that she was entitled to, rule 43.1(b) which was quoted earlier.

Robert, I’m also stuck on “compelled”. 

Merriam-Webster...

Compel
1: to drive or urge forcefully or irresistibly

2: to cause to do or occur by overwhelming pressure

In the US, we have US Appeal 2.  I think Yellow had options to avoid Green which did not break rule 11 v Blue.

“When PW failed to respond to PL’s hail for room to tack, PL was faced with the necessity of taking alternative action to avoid S. This raises the question of whether she was exonerated by rule 43.1(a) as the innocent victim of another boat’s breach. We think not, since the protest committee found that PL could have gone astern of S after it became clear that PW was not giving her room to tack as required by rule 20.2(c). A boat breaking a rule is not exonerated by rule 43.1(a) unless she was compelled by another boat to break a rule.”