Rules that apply may include RRS 10, 13, 15, 16, and 18.3.  I note that the diagram shows Green still above lay-line, but the stated facts dispute this point.

Facts Found:

1. At position (b), Red is port keep-clear boat w.r.t. Green (RRS 10) AND

2. Also at position (b), Green has just acquired ROW, and owes Red room to keep clear (RRS 15)

To parse the Fact 2 above, I would ask green and red several questions about timing. Moving forward with assumed fact 3 and discussion:
3. Red began their tack to avoid promptly once Green reached close-hauled. (Green suggests Red should have continued to sail straight another boatlength to avoid, but Red felt they were doing the most prudent thing to try to avoid and were expecting Green to luff up instead of duck (as OP alludes to).)

4. Green then creates the room they owe vis-a-vis RRS 15 by bearing away and ducking Red (step between b and c)

5. Green continues to give room to keep clear to Red due to RRS 15 and RRS 16 while Red is tacking

6. Green never becomes overlapped inside of Red -- per image 1

7. Red allows 1.5 boatwidth between her and the mark at position (c) -- per image 1

8. Green cannot fetch the mark after her duck -- per OP's discussion

Conclusion:

One question I have on the scenario would be how should the circle extend. Assuming the boats are close together that no one is free to move does boat 3 get to tack first despite being the most outside boat and stopping boat 1 the inside most boat from getting around because they have to tack early to avoid the starboard boat. It's my understanding that the circle should extend similar to a leeward mark with overlap having been established boat 3 and 2 can not go until 1 can tack and make the mark

This diagram is an example of 18.3(b). The outside overlapped boat that tacked shall give mark room to the inside boat that entered the zone on stbd.

If Green then failed to fetch the mark, we need another diagram to show the position after the mark.

John

Craig:   we're unsure when the moment of the tack and the foul coincide.

I'm not quite clear what you mean, but I think you are saying that we're not sure whether Green changed course to give Red room to keep clear before or after Red passed head to wind.

It certainly makes a difference with respect to RRS 18.3.

As diagrammed and described, it looks to me clear that Green bore away while Red was still on port tack and had not yet passed head to wind.  In that case, when Red does pass head to wind RRS 18.3 does not apply, not just because Green ducked, but because, as a result of the duck, Green was not fetching the mark when Red passed head to wind.

If there was any doubt a protest committee should resolve that by applying the last point of certainty, that is, that R and G were on opposite tacks when Green changed course. 

Nick, I think you are introducing some irrelevant considerations and you could tighten up your analysis a bit. 

Your FF 3 about why Red tacked is irrelevant and I don’t think it is consistent with OP who said ‘3. Boats 1&2 both tack to round’.  I understand this to mean that Red tacked in order to round the mark, not in an attempt to keep clear of Green.  Either way, I don’t think it matters.  What boats ‘suggest’ or ‘feel’ don’t have much place in a protest committee’s decision. The rules are about what boats actually do, not how they think or feel, nor, in most cases, why they do things. 

We wouldn’t usually refer to RRS 16 when a boat was changing courses to give room to keep clear in accordance with RRS 15. 

When RRS 15 applies and the right of way boat changes course to give room to keep clear, we would usually conclude that the right of way boat gave room to keep clear and the give way boat kept clear:  no rule broken, so there is no need to consider exoneration. 

If you say that a right-of-way boat gave room to keep clear and the give way boat is exonerated for not keeping clear, wouldn't that mean that she wasn't given room to keep clear? 

I certainly agree that at the last position in Craig’s diagram Green is clear astern of Red and RRS 18.2(c) does not apply, but I think it’s inevitable that shortly after that position, Red will bear away around the mark and become overlapped outside Green, and required to give Green mark-room.  But you’re probably right:  if Green isn’t fetching, then she will never get lose enough to Red for there to be a question of Red not giving her mark-room. 

In my humble opinion - take the mark and Blue boat away from the picture. The thing that remains then is Red and Green on port tack. Green is making a clear (sharp) tack and acquires starboard rights on Red (RRS10) . In the picture Red is making a slow tack (wide curve) and Green has to duck to avoic collission (RRS14). The only discussion that remains here is whether Green allowed Red enough time to stay clear (RRS13). Possibly Green could have hailed Red upfront that she was tacking and Red could have reacted sooner. If these are manouevrable dinghies I would say Red is in the wrong here - RRS 10. In case of big boats, multiple crew and slower reaction time then RRS13 would apply and Green is in the wrong. 

The Mark, RRS 18 and the Blue boat are irrelevant to the situation between Red and Green (imho) and only confuse.

John Allan,
My FF3 is an assumption, so I'm telling you why I landed there... I wouldn't write out the italic parts. I based it on the quote "but rather than go above close-hauled, she ducks"

Point taken on RRS 15/16... I won't refer to 16 while 15 is still in effect.

What is the point of 43.1 if not for this situation? We've been over this before, but 15 and 16 should say "make room" in that there was not initially room, but it was shortly thereafter created.
(I think if you want to discuss that last item, an email might be better than derailing this thread.)

 James Mercer One question I have on the scenario would be how should the circle extend.

Like this?

No.

Green is the same tack leeward boat, entering the zone clear ahead of both Red and Blue.  Green has right of way (RRS 12) and is entitled to mark-room (RRS 18.2(a)(2)).

Green may luff up and attack Red/Blue, and as long as she does not get outside the mark-room to which she is entitled, that is, initially, the direct corridor to the mark (Case 75), she will be exonerated for breaking RRS 16 by RRS 43.1(b), but if she passes head to wind (tacks) the following things will happen:

See  Case 15 

Disregarding Green, Red is the same tack leeward boat on Blue, entering the zone overlapped outside Blue  Red has right of way over Blue (RRS 11), but is required to give Blue mark-room (RRS (18.2(a)(1)).

Red can luff up and attack Blue while they are outside the zone, subject only giving Blue room to keep clear in accordance with RRS 16.1, but once the first of them reaches the zone, any luff by Red will probably be denying Blue her mark-room and breaking RRS 18.2(a)(1).

Blue is required to keep clear of Red but is entitled to mark-room.  Once Blue reaches a position where she can round the mark, she is entitled to do that, that is to say, she is entitled to room to luff up around the mark (including room for her stern to kick out, or to release a headsail to facilitate luffing to windward) UNTIL she passes head to wind, at which point /RRS 18 will cease to apply (RRS 18.1(a)(1) or (2)) and Blue will lose her entitlement to mark-room and to any exoneration under RRS 43.1(b).

Going back to Craig's original diagram, the crux of the scenario is that when Green tacks onto starboard, she is so close to Red that her RRS 15 obligation to initially give room means that Red/Blue don't need to tack away early.  If G had been coming in on starboard from 2 or 3 hull lengths further out then she would have had Red and Blue on toast and could have bounced them away from the layline.

Yes that's right:  Blue is pinning Red and Green out. 

Just as a tactical comment:  Green has nobody but herself to blame for being a marshmallow.  Any time she could have closed up the lee bow and squished Red out the back and got ahead and to windward with freedom to tack.