Who's right of way at Leeward Mark?

Under the 2009 RRS, right of way does not change near marks.  What you get is exoneration under rule 18.5 if you do not keep clear when a right of way rule (rules 10, 11, 12, 13) requires it.

It is very doubtful that an opposite tack boat approaching a leeward mark close enough for there to be an incident will be clear astern at the zone.  She is almost certain to be overlapped.  See the diagram

But it makes very little difference whether you are clear ahead or overlapped inside at the zone, the outside port tacker (O) must still give you (I) mark-room under rule 18.2(b).  This is in addition to keeping clear under rules 10, 11, and 12.

There is no change in right of way in this scenario.  Applicable rules change, but right of way does not.  Initially, O on port must keep clear of I on starboard (rule 10).  When I gybes, O will momentarily come clear astern and must keep clear of I clear ahead (rule 12), then I will become overlapped to leeward and O must keep clear (rule 11).

While I is sailing to the mark, and until I is at the mark, if I changes course, I must give O room to keep clear under rule 16.

Once I is at the mark, I may change course to sail her proper course at the mark and I will be exonerated under rule 18.5 if she does not give O room to keep clear.

In the case described, O kept clear of I therefore I did not fail to give O room to keep clear.

O is always obliged under rule 14 to avoid contact if reasonably possible.

Did you hail 'protest', fly a red flag (if necessary) and follow through with a protest?  If not O will never learn.

To understand better about to and at the mark, read about the roundabout analogy here

http://www.yachting.org.au/site/yachting/ayf/downloads/Techn ical/RaceManagement/Comparison%20of%20Old%20and%20New%20Sect ion%20C%20Rules.pdf

A right of way boat is not obliged to begin to take avoiding action to keep clear of another boat that will acquire right of way before that boat does acquire right of way.  A boat that has right of way is not obliged to anticipate that another boat will break the rules (Case 27).  A boat entitled to right of way or room or mark-room need not act to avoid contact until it is clear that the other boat is not keeping clear or giving room or mark-room (rule 14(a)).  Otherwise, of course a boat that is obliged to keep clear of you is obliged to anticipate.  Read the definition of keep clear.

You are entitled to start changing course to round a mark any time you please as long as, if you are not yet at the mark, you give a boat required to keep clear of you, room to keep clear when you change course.

It may seem a bit circular, but once you are close enough to the mark that any sane person would be changing course to get around it, you are pretty much at the mark.  Maybe,to put it another way, once your proper course is to change course around the mark, you are at it.  I reckon that once you are inside one boat length of the mark, you are at it.

So, I think it is fair to say, in this case, once you reach the point on Stbd, where your proper course is to gybe and sail round the mark, you are entitled to bang through the gybe and slam the door in O's face as hard as you like, as long as you don't hit her.

Of course, if the act of keeping clear for the other boat involves them going into or the wrong side of the mark, then that is their hard luck. Maybe they will plan better next time... 

Edit - I guess if the mark is large and solid and can cause damage, they would have to be given room to avoid it?

No.

19.1 When Rule 19 Applies

Rule 19 applies between boats at an obstruction except when it is also a mark the boats are required to leave on the same side. However, at a continuing obstruction, rule 19 always applies and rule 18 does not.

So, unless the mark is big enough to be a continuing obstruction, a boat owing another mark-room that gets inside has absolutely no right to room while the entitled boat is sailing her proper course at the mark.

A generous minded protest committee might allow that a ginormous navigation mark or a big pile structure was a continuing obstruction say to a tiny little optimist, but if it's surrounded by navigable water, then there's not much case.