Two port tack boats in the zone need to tack to round the mark to port. I am Just follow the mark room rules though here. 

1 Two boats enter the zone on port. 18.2a  ON

2 IF Boat entitled to make-room passes head to wind, 18.2a OFF by rule 18.2b and 18.1A(1) ON All of rule 18 is OFF.

  Rule 10 ON.

 
OR 
  IF other boat tacks 18.2a OFF and 18.1A(1) ON All of rule 18 is OFF.  SAME RESULT 

Rule 10 ON.

3 Other boat tacks in the zone 18.1A(1) OFF = Rule 18 is back on

 as 18.2(C) ON.

"18.2(c) When rule 18.2(a) does not apply and the boats are overlapped,the outside boat shall give the inside boat mark-room.”

So the result after both boats have tacked from port to starboard the inside boat has the Mark Room.  

• If the first boat to tack was fetching the mark, RRS 18.2 is disapplied by RRS 18.3, but the obligations of RRS 18.3(a) and (b) do not apply because neither boat was on starboard tack since entering the zone:  no entitlements or obligations under RRS 18 apply.
• If the first boat to tack was not fetching the mark, RRS 18.3 is not engaged and RRS 18.2(c) applies to require whichever boat is overlapped outside to give mark-room.

1. A boat passes head to wind from port to starboard tack in the zone of a mark to be left to port;
2. Another boat is on starboard tack and fetching the mark;  and
3. The other boat has been on starboard tack since entering the zone.

The rule is now to be applied in two steps: 

1.      Whether the other boat is on starboard tack and fetching the mark, in which case RRS 18.2 does not apply, and 
2.      Whether the other boat has been fetching the mark since entering the zone, in which case the tacking boat is required to: 
(a)    not cause her to sail above close hauled to avoid contact, or 
(b)   if overlapped outside her, to give her mark-room. 

1. Magenta (M) tacks inside zone (position 1.6ish)
2. Teal (T) also tacks inside zone (pos 2)
3. M had a better tack and becomes overlapped inside/to leeward of T (pos 3)

• Because T tacked inside zone and M was already on starboard, 18.2 does NOT apply between them
• Because M also tacked inside the zone, the 2nd part of 18.3* does not apply between them.

*Whether the other boat has been fetching the mark since entering the zone, in which case the tacking boat is required to: 
(a)    not cause her to sail above close hauled to avoid contact, or 
(b)   if overlapped outside her, to give her mark-room. 

• At position 3, their only relationship is windward/leeward with 15 and 16 active, right?
• Anything I'm missing with that last point? There's no mark-room at all, right?
• If M feels she cannot comply with both 15/16 and 31, should she "bail" by gybing around to avoid a foul?

  

 Mark Townsend said Tack, Starboard or Port A boat is on the tack, starboard or port, corresponding to her windward side. 

• she can fall behind Green, either by dwelling in her tack, or by easing sheets and slowing, and follow Green around;
• Once overlapped on Green, and having initially given Green room to keep clear, she can stand on a steady course and drive Green past the mark before gybing inside her.

18.3 Tacking in the Zone
If a boat passes head to wind from port to starboard tack in the zone of a mark to be left to port, rule 18.2 does not apply between her and another boat on starboard tack that is fetching the mark. If the other boat has been on starboard tack since entering the zone, the boat that passed head to wind:
(a)shall not cause the other boat to sail above close-hauled to avoid contact, and
(b)shall give mark-room if the other boat becomes overlapped inside her.

Fetching  A boat is fetching a mark when she is in a position to pass to windward of it and leave it on the required side without changing tack.

1. Inside passes HTW before outside
2. Outside passes HTW before inside
3. They pass HTW exactly at the same time. 

1. 18.1(a)(1) turned off 18. 
2. When Outside passes HTW, 18.3 applies
   1. Inside is already on starboard and is fetching the mark.
3. 18.1(a) turns 18 back on. 
4. 18.3 states 18.2 does not apply
5. Inside didn't enter on starboard, therefore 18.3(b) does not apply
6. No MR for Inside or Outside

1. 18.1(a)(1) turned off 18. 
2. When Inside passes HTW, 18.3 applies
   1. Outside is already on starboard and is fetching the mark. 
3. 18.1(a) turns 18 back on. 
4. 18.3 states 18.2 does not apply
5. Outside didn't enter on starboard, therefore 18.3(b) does not apply
6. No MR for Inside or Outside. 

1. A boat is on the previous tack up to and including the point of HTW (def: Leeward and Windward)
2. At the moment that the boats pass HTW..
   1. 18.2(b) turns off MR for Inside
   2. From the POV of Outside, when Outside passes HTW, Inside is also passed HTW on starboard and is fetching the mark, so 18.3 applies
   3. From the POV of Inside, when Inside passes HTW, Outside is also passed HTW on starboard and is fetching the mark, so 18.3 applies
3. 18.3 states 18.2 does not apply
4. Neither boat entered on starboard, therefore 18.3(b) does not apply
5. No MR for Inside or Outside. 

Fetching  A boat is fetching a mark when she is in a position to pass to windward of it and leave it on the required side without changing tack.