In my opinion 

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.  

You could add in the other right of rules 13/15/16 depending how close the tacking was but the questions are about Mark Room.  

   

Here's a presentation I gave yesterday.

It's in the context of a review of rules generally and the major changes affecting sailors on the water.

Email me if you want the Powerpoint original.

Robin ... I think you are correct ... my error. 

Start is out of finish ... so we are back to where we were before. We can change 28, remove the reference to "sail the course" and write a new 28. 

Yes .. but those items already listed under def:rules (except "other docs") wouldn't need to be as long as the RRS is invoked.  In either case, the NOR/SI changing A5 will point the way for sailors to find it. 

OK, leaving aside the evidential difficulties, there was a compliance difficulty in the past if one boat was moving fast and then expecting room after turning hard and arriving within three boat lengths. The fact that we now need to decide the order of operations in advance is of value to the game. It's generally a good thing if rules designed to avoid collisions also force people to slow down a little and approach in and ordered manner with conversations occurring in advance of arriving at the zone.

No disagreement that it has changed ever so slightly, but the result should be a slower and more tactical approach to the zone, rather than additional confusion.