There is no point at which green has mark room.
She is never the inside boat in the rotation around the mark.

Agree that green is never the inside boat. The question is:  is green required to provide yellow mark room to finish after both tack in the zone? What prevents green from sailing past yellows layline and forcing them to tack behind? 

I think Philip is right. Although it looks like Green is "inside" of Yellow at 2 & 3, Yellow is actually the inside boat for a port rounding. 

Yellow would initially be entitled to mark-room (as well as ROW) if she's either clear ahead or overlapped when the first of them reaches the zone. Yellow's entitlement to mark room does not include room to tack so if she wants to tack she must keep clear of Green. Yellow loses her entitlement to mark-room at 3 when she passes head to wind in the zone.

At 4 Green is ROW but Yellow is again inside overlapped and entitled to mark-room by 18.2(a). At this point Yellow is overlapped inside & to windward so her mark-room includes room to tack back onto starboard to fetch the mark.

From the diagram at 5 it looks like Green did not give Yellow mark-room or room to keep clear and forced Yellow to break 31.

"What prevents green from sailing past yellows layline and forcing them to tack behind?"

I'd say what prevents that is Yellow luffing Green up to head to wind, and we can both sit here until Green decides to act right and tack away.

Yellow is inside and overlapped to green once they reach the zone so 18.2a is applicable, therefore yellow is entitled to mark-room.  18.2 b only comes into play if green was clear ahead as they entered the zone. 18.2 d would be applicable if yellow had to tack but green did not.

When the boats reach the zone, yellow gains mark room as the inside boat of the rounding and is entitled to sail straight to the mark - even straight upwind - and green must keep clear.
Green may be forced to tack away in order to keep clear.
If Yellow passes head-to-wind, initial mark room ends.
There may be a point during the tacking at which Yellow is on port and Green is still on starboard. Yellow must keep clear.
After the tack, Yellow, if overlapped, is again the inside overlapped boat with mark room.

Is this correct even though Yellow's straight line to the mark would require a tack (since Yellow's point of entry to the zone is further left than the mark), and the definition of mark room specifically excludes room to tack?

If so, does this mean Yellow effectively gains luffing rights upon entering the zone by virtue of Rule 18, even if the overlap was established from behind and Yellow was restricted to proper course before then, and even if Yellow can't fetch the mark on port tack despite luffing?

Sam, at 1 Yellow is leeward so can luff up to head to wind whenever she wants, whether in the zone or not. If she becomes clear ahead she can still luff up to HTW without losing ROW, forcing Green to keep clear.

At 4 Green is leeward ROW but her right to luff Yellow is limited by her obligation to give mark-room

Thanks, all.  You see I was struggling to figure which boat is 'inside'.  It's not apparent to the eye.  I think they could stand to put that one in the definitions.  And it seems that there is nothing to require green to make room for yellow to tack.  Once they're on port tack though, there is no need to tack back to starboard, but yellow would get room to do so once she can fetch the mark.

I realize that one thing that IS different than an upwind rounding mark is that the boats won't be rounding the mark, so there is no need to fetch it.
-Vince

Thanks, Tim, but I was wondering about the interplay between this scenario and Rule 17, if Yellow had established overlap from clear astern and thus could not sail above her proper course.  Does her right to mark room under Rule 18 override that?  Or - even regardless of Rule 18 - does she have a right to luff head to wind once her proper course in the absence of Green would be to tack to the finish?

Mark room ends when the boat with mark room goes beyond head to wind. Period. Full stop.
She may gain new mark room later. However:

In the present case, Green very likely was able to give mark room after the tacks at position 4.
At position 5, Green luffed Yellow into the mark and failed to give mark room.

Partially answering my own question: Team Race Call D7 (attached) makes clear that a leeward boat's proper course is to luff once she is past layline to a mark.  It's still unclear to me whether Rule 18 would give Yellow the right to luff before she reaches layline to the finish mark, if she is otherwise restricted by Rule 17 and can't fetch the mark without tacking.

Here's a weirder version:

For whatever reason, Yellow doesn't luff here, and Blue takes her past layline to finish.  Yellow gybes out as Blue tacks.  Because Blue is past the mark and needs to head down to finish, and let's say Yellow executes her gybe better than Blue executes her tack, they would collide if they both try to finish tight at the pin.

My understanding is Yellow's mark room under 18.2(b) from entering the zone is still valid because she never passed head to wind.  Blue does not have mark room under 18.2(a) because 18.2(b) applies.  I believe Blue fouls here if she attempts to finish ahead of Yellow (rather than avoiding Yellow by tacking again or slowing until Yellow finishes).  Is that correct?

(Phillip - I accidentally "disliked" your recent post.  There doesn't seem to be a way for me to undo it.  Sorry about that, it's a fine post!)

Sam, I'd be interested in other perspectives but my take on the question of rule 17 limitations upwind would be this: If rule 17 is on and at any point on the leg the leeward boat decided that her proper course was to tack (perhaps due to a wind shift or other tactical considerations that don't include the windward boat), leeward would be entitled to alter toward her proper course until some other rule stopped her. So if leeward wanted to tack she could come up as high as head to wind and force windward to do likewise. If either boat passes head to wind then 17 is off and we have a new ROW scenario. I still don't think that being in the zone or not makes a difference.

One reason I figure that 18 doesn't enter into it is that if you slide the mark to the right Green could still be sailing Yellow past the layline well outside the zone. In that instance it's clear that Yellow's proper course is to luff and tack. So I think even with 17 on Green would have to respond to the luff until both boats passed HTW and Green gained ROW.

It also occurs to me that in the scenario in the original post at position 2 Yellow could dip down slightly, break the overlap turning 17 off, and then turn back to close hauled with a new overlap and full luffing rights.

Sam, in your latest diagram, it would be difficult to understand that Yellow could complete her gybe at exactly the same time as Blue passes head to wind.  Therefore, at some time during the manoeuvre, it is conceivable that the boats were on opposite tacks for a brief period.  As they are both still on a beat to windward, rule 18 would switch off (rule 18.1(a)).  At position 5, rule 18.2(a) applies, giving Blue mark room.

In the second diagram, rule 18.2c1 applies. Yellow still has mark room. Team racing call E10 q4 deals with the case where 18 is turned off by the boat without mark room tacking then being turned on again. 

Nobody is talking about the last sentence of RRS 18.1. Why doesn’t it turn-off rule 18 between Y and G just after position #3?

Angelo, I don't understand how Mark-Room may have been given by that position.  Can you please explain your thoughts?
Along with my lack of intuition in this case about which is the 'inside' boat, I have problems understanding how green meets the requirements of having given Mark-Room, (or maybe even giving Mark-Room) before both boats tack.  Particularly, I don't understand how Green gives "room to sail to the mark when her proper course is to sail close to it", from Mark-Room definition (a).  I think Yellow's proper course might likely be to tack shortly before position 3.  I'm surprised at how complicated this has become.  

Shortly after position 3 Green and then Yellow passes head to wind. When the first boat passes head to wind rule 18 no longer applies. See 18.1(a) the boats are on opposite tacks on a beat to windward. When the second boat passes through head to wind rule 18.2(a) and not 18.2(b) applies. Yellow again has mark-room. Yellow's challenge is to get Green to pass head to wind first, otherwise Yellow breaks rule 13.

Vince, I wasn’t suggesting MR was given, I just wanted to get folk thinking about it and satisfying for themselves that it wasn’t. 

Sorry, Angelo!  I realized afterward that I shouldn't have inferred that you were thinking so.  I've been reading here for a while and have come to think that several of you know all the answers.  :-)
Over recent months of study, one of my main areas of uncertainty is about when "mark-room has been given".

Mark:  Understood; except that I would say that as long as Yellow keeps clear of Green until she's on a close-hauled course, she doesn't break 13.  Right?

This occurred two weeks ago between two Lasers.  I didn't witness it, but this is how I understood it happened. The most important aspect was that there were new mark-room rights after the tack, and green didn't give yellow room to pass the mark on the correct side, forcing her to break 31.  But the rights and obligations before the tack are not so clear (to me).  Definitely gives rise to some thinking!

That said, I think Tim Hohmann is correct when he says "So if leeward wanted to tack she could come up as high as head to wind and force windward to do likewise".  The definition of mark-room for a boat doesn't include room to tack unless she is inside and to windward.  I understand that the consensus is that yellow is inside, but she isn't to windward,  so she can't tack.
Thank you all.  Interesting!  
-Vince

Vincent, Mark T’s straightforward answer is the correct solution.  That said, there are interesting questions. 

One way to look at it is to ask, “what mark-room was Yellow entitled to when they first entered the zone?”  

Yellow’s ability to “sail to the mark” is functionally limited to Yellow turning HTW after rule 18 applies.  When you see the wind direction and where Yellow enters the zone, Yellow enters already past where turning HTW would take her to the mark on its proper side (assuming there isn’t a current from bottom left to top right that would push her there). 

Q1: Is Yellow already sailing outside the mark-room she is entitled to as soon as she is entitled to it?  If Green was closer to where Yellow is at position 3 and Yellow broke rule 16 at 3, would she be exonerated under 43.1(b)?

Q2: If Green did nothing to prevent Yellow from sailing in the mark-room Yellow was entitled to, why don’t we say the mark-room Yellow was entitled to was given by Green?  Is it even possible for Green to “give” Yellow mark-room when they enter the zone?

Slide your scenario up 1 layline and the question changes, as now it’s is clear Yellow heading HTW after 18 turns on could take her both to the mark and on its proper side.