Yes - as both boats tacked in the zone on a beat to windward, R 18.2(a) applies and the outside boat gives the inside boat mark room. And No - mark room was not given as the outside boat luffed the inside boat into the mark. Alternatively, the leeward boat with ROW broke R 16,1 when she luffed hard.
John 

R10 - 24 stb, 89 port
R16.1, 24 ROW changing course
R10 - 24 S, 89 P
18.2(a) 89 overlapped inside 24
Penalise 24

Doesn't mark room end the moment her hull touches the finish line? She has sailed the course and no longer has a PC.

I don't think so - she is still racing until she clears the finishing line and marks ans so all the rules apply. See definition of Racing.
John

But look at the definitions.
 Finish 
A boat finishes when, after starting, any part of her hull crosses the finishing line from the course side. 

 Mark-Room 
Room for a boat to leave a mark on the required side. Also, 
(a) room to sail to the mark when her proper course is to sail close to it, and 
(b) room to round or pass the mark as necessary to sail the course without touching the mark. 

 Sail the Course 
A boat sails the course provided that a string representing her track from the time she begins to approach the starting line from its prestart side to start until she finishes, when drawn taut 

When her hull touches the line :
She has finished, 
She has no PC so cannot sail to the mark,
She has sailed the course so has had room as necessary to do it.

The only thing she hasn't had is leave the mark on the required side but there is no longer a required side.

Why should she be entitled to mark room after it has all been given.

Once a boat finishes, she still has to clear the finish line and its marks.  If she were to touch a finish mark after  "any part of her hull crosses the finishing line from the course side" and before she has cleared the finish line and its marks, she would infringe rule 31 while still racing and should take a penalty.  Therefore, she must be given room at the mark while she is still racing, even if she may have finished
Murray

FWIW, I found errors in my above analysis regarding the application of mark-room once Boat 24 finished.  I've edited my play-by-play with strikethroughs and [bolded additions in brackets].

In summary, once Boat 24 finishes, the finish-mark no longer has a required-side for Boat 24 and therefore the 1st sentence of 18.1 cannot be satisfied thereafter.

That said, as the ROW boat under rule 11 and room-giving under rule 16.1, as Boat 24 continues to change course, the room she owes Boat 89 includes room for Boat 89 to comply with rule 31. (the point Richard was making).

Ang,

You said

I don't think that's the best construction you can put on the rules.

Rule 18 applies until boats leave the mark.

89 has not left the mark.

By your interpretation, had 24, tacking into the leeward ovelap position in the zone, held a steady course close to the mark, allowing 89 just enough room to get her bow across the line, but then unavoidably touching the mark (supposing the mark was not an obstruction) would break no rule.

I think 24's obligation includes giving 89 room to leave the mark without touching it.

John, re: “I don't think that's the best construction you can put on the rules.”

As soon as Boat 24’s bow crosses the finish line, she could have fell-off before tacking, gybed around and passed [left] the mark on the starboard side. Once the boat finishes, she has sailed the course.

“Rule 18 applies between boats when they are required to leave a mark on the same side…”. 

Boat 89 still gets room to not touch the mark (rule 31) under rule 16.1 because Boat 24, a ROW, altered course.  It’s sort’of “mark-room lite”. 

John, BTW, I’d be happy if you can formulate a way to tie the rules together to find a “required side” once a boat has finished. 

I know you understand, but just to state it for other’s benefit, “required side” in 18.1 refers to the ‘side of the boat’ not the mark. 

Boat 24 could have fell-off on port after her nose crossed the line (without clearing the line)  and headed away from the line on the course-side (finally clearing it)  … leaving the mark on her starboard side. 

By the same logic, in the attached diagram, Yellow has finished and, as rule 18 no longer applies, is not required to give Blue room.  I don't think so!

Murray, I don’t like it either. The challenge is to explain it using the rules and cases as written to support how this rule has been interpreted and applied. 

R28.1 clearly supports that a boat can leave on the opposite side once she has finished. She can pop her bow through the line, get the hoot then immediately turn round and sail round the other side of the mark.

 28.1 A boat shall start, sail the course and then finish. While doing so, she may leave on either side a mark that does not begin, bound or end the leg she is sailing. After finishing she need not cross the finishing line completely.

It is a nasty trap as Murray shows. Blue will either hit the mark or pass the wrong side then take a penalty turn if needed and then sail back to refinish. 

It's possible R23.2 'Interfering with a boat sailing another leg' could apply between yellow finishing and blue finishing.

It's possible that the mark could become an obstruction once both have finished and it cannot be passed safely. So R19 could possibly apply.

There are a few nasty holes in the rules which suddenly leave a boat with no protection such as a leeward bow out boat overlapped with a windward boat who becomes overlapped with a previously clear ahead boat. The middle boat is protected by R19 while both are clear astern but as soon as leeward becomes overlapped R19 switches off.  

I read the first sentence of 18.1 as a condition for when rule 18 can be turned on, but not as a condition for when rule 18 is turned off.

Rule 18.2(d) tends to support that stance, Johan.

(d) Rules 18.2(b) and (c) cease to apply if the boat entitled to markroom passes head to wind or leaves the zone.

If the boat entitled to markroom passes head to wind, she would be on the opposite tack than the other boat (boats must have been on same tack prior in order for 18 to be turned on).
However, rule 18.2(d) clearly indicates that only 18.2(b) and (c) are turned off.  How could the other parts of rule 18 not be turned off if rule 18 is turned off?

Yet, Case 95 tells us that rule 18 turns off when boats are on opposite tacks on a beat.  What, then, is the purpose of the wording in rule 18.2(d)?

If rule 18 can be turned off by any of the conditions of 18.1(a) though (d), then surely it can also be turned off when boats are no longer "required" to leave the mark on the same side.
In the diagram above, is Yellow 'required' to leave the mark on the same side as Blue?  
The Cambridge dictionary  defines 'required' as "necessary according to the rules or for a particular purpose".  
While she may no longer be required by the rules to leave the mark to port, in the current situation it is necessary for Yellow to leave the mark to port in order to clear the finish line.  Therefore Rule 18 applies.
T
Murray

OK .. to avoid confusion, I deleted my edited post above and I'm reposting this with my original play-by-play.  Allow me address the 18.1 issue a bit (after some offline discussions). 

Parsing the 1st sentence of 18.1 

There are 2 items in this sentence. 

The issue is that the 1st part and the 2nd part are not evaluated the same way. "Are" is the plural of "is", so this difference isn't apparent. 

The 2nd part of this sentence is tested, if it's true or not, moment-by-moment based upon the current positions of the boats.  One boat enters the zone, the test is true .. both boats leave the zone, the test is false.  At a single meeting-of-boats at a mark, we are constantly testing this condition and it can turn on and off more than once.

However, "are required to leave" part of the sentence is not evaluated moment-by-moment, but it describes a more timeless state of the boats based upon what is required of them by their race documents/course-description .. and not what is required of them at that moment. What is required of them by the race docs/course does not change based upon where there are at any single moment.  

IMO, 18.1 could be worded more clearly to indicate the above.

So .. with that in hand, below is a clean play-by-play

-------------------------------------------------------------------------

Assuming we are looking directly down the finish-line ... and starting from this moment where both boats had just tacked inside the zone.

#1) Rules: 11 and 18.2(a)

#2) Rules: 11, 16.1 and 18.2(a)

#3) Boat 24 has finished, but is still racing
Rules: 11, 16.1, 18.2(a) and def: racing

#4) Boat 89 finishes.  Both boats are still racing.
Rules 11, 16.1, 18.2(a) and def: racing

#5) Both boats are still racing.  Boat 24 passes head to wind.  Boats are on opposite tacks thus 18.2(a) is off.  Boat 24 must keep clear of Boat 89
Rules: 13 and def: racing ..

.

#6) Both boats are still racing.  Boat 89 alters course to windward and then later passes HTW to avoid Boat 24
Rules: 13, 14, def: racing .. and 18.2(a) turns back on as soon as Boat 89 passes HTW.

After Boat 89 passes HTW.. both are under 13 until they reach their close-hauled courses (which they reach almost simultaneously).  While rule 13 applies to both boats, Boat 89 being to port shall keep clear of Boat 24. Simultaneously, Boat 24 owes Boat 89 mark-room, [and room to keep clear under 16.1 as the ROW boat changing course].

#7) Both boats are no longer racing.  There is no contact between the boats or marks.


Conclusion:
After passing head to wind and before being on a close-hauled course, Boat 24 failed to keep clear of Boat 89 and broke RRS 13.

Here is an interesting additional question.  Looking at #6 above, does rule 15 apply to Boat 24 at the moment Boat 89 passes HTW?

Just after #6, both boats are subject to rule 13 after Boat 89 passes HTW.
At that moment, Boat 89 shall keep clear of Boat 24 under rule 13 as she is to port of Boat 24.  This means Boat 24 acquires ROW at that moment.
I found above that Boat 89 was forced to tack to avoid contact with Boat 24, who broke rule 13.

Did Boat 24 acquire ROW under rule 13 because of the Boat 89's actions or her own?

Ang, I think your last line should be Did boat 24 acquire ROW... not 89.... 

24 tacked 89 ROW doesn't change
89 tacked ROW passes to 24 R13 2nd part. R15 applies due to 89's own action of tacking.

However, 24's breach of 13 compelled 89 to tack so 89 is exonerated by 43.1a for any breach of R15. 

Nice catch on typo Richard.  Thanks. 

Re: “24 tacked 89 ROW doesn't change”, before 24 tacked, wasn’t 24 ROW by R11?