The 200 is entitled to Mark Room as per rule 18.2b as she is clear ahead.

As soon as she passed head to wind her right to Mark Room ceases as per 18.2c plus she was no longer at the Mark (as you stated she was 1BL past the mark). 

Mark room only includes room to tack is the boat is to windward, overlapped and inside at the mark (as per the definition of Mark Room). So she did not have the room to tack as part of the Mark Room obligations.

It would hinge on 3 things for me were I on a PC:

1) What was the separation between the 2 boats

2) Has the 200 completed her tack before the Laser started her mark rounding?

if so:

3) Would the Laser have passed clear astern had she not altered course (i.e. rounded the mark)

My gut feeling is that the boats were close, the 200 slid off wide due to a poor rounding and tacked but the Laser was already sailing higher so the 200 would get pinged.

200 reached the zone clear ahead of L on the same tack.

1. L continues to be required to keep clear of 200, but now under rule 11;
2. L continues to be required to give 200 mark-room, (but if 200 has left the mark astern without interference from L, then L has given 200 all the mark-room she needs;
3. L is required to give 200 room to sail her proper course (rule 18.2( c )(2)), and will continue to be required to give that room while they remain overlapped [and rule 18 continues to apply].

If so:

Unless of course there was a witness.....

I guess that when I round a mark to starboard from a fetch to a beat with someone close behind I am looking to do a very tidy luff and tack.  My guess is that RS200 was trying to prevent a possible overlap, which prevented a good wide in - tight out rounding (not allowed in a "mark room" situation now I know).  

I shall follow the thread with interest, but I would have thought that if the RS200 misses the opportunity to do a tidy luff and snappy tack in the mark room lane, it will prove best to keep on the same tack whilst looking over his shoulder and postpone tacking until the Laser puts his helm down.  A bit of windward heel and dirty air from the 200 would have encouraged the Laser to either bear off behind or tack ASAP.

Is it a lie if they believe it to be true? Good luck proving that and proving any deliberate attempt to deceive....

So pretty good evidence that tacking was 200's proper course.

So the 200 was right of way boat, entitled to room to sail her proper course (that is, luff into her tack), and exoneration if she broke rule 16 while doing so, until she passed head to wind.

As I said before, given that 200's proper course was to tack, she was entitled to room to sail her proper course (rule 18.2( c )(2)), and while doing so was exonerated is she failed to give L room to keep clear under rule 21.

Once 200 passed head to wind rule 13 began to apply and she was required to keep clear of L.

If L needed to change course to avoid 200 then 200 broke rule 13.

The rule 21 exoneration is really just a bonus for 200:  this is probably best looked at as simply a boat clear ahead and a little to leeward or just on the line of the astern boat having enough space to tack without breaking rule 13 (or when tacking from starboard onto port, then rule 10).

Surely if the Laser which had been forced to tack managed to fit between the RS200 and the mark without hitting either, the RS200 would find it quite hard to prove that he was sailing a proper course?

Correct me if I am wrong but I would have thought that a proper course was either to tack immediately after passing the mark, or carrying on on the same tack ... holding on leaving enough room for a boat to fit in between you and the mark would surely be stretching the credibility of what was a proper course?

200's proper course is irrelevant once she passes head to wind and rule 13 applies.

No, 

Mark-room does not include room to tack in this case because 200 is overlapped outside, not inside L (Definition:  Mark-room last sentence).

In any case, even when mark-room includes room to tack, this is effectively limited to her change of course until the boat is head to wind:  once past head to wind, the boat's entitlement to mark-room of any kind (and her entitlement to room under rule 18.2( c )(2)) shuts off because rule 18 in its entirety ceases to apply.

So effectively, all a boat gets out of an entitlement to room to tack as part of mark-room is room for her stern to kick out in a tack.

In a scenario as described it would be extraordinary for L to rely on a protest solely about rule 16:  she would be crazy if her protest wasn't primarily based on rule 13.

And while 200 has two defences against rule 16 (that she didn't fail to give room to keep clear at all, because L tacked away and there wasn't contact, AND that, if she did break rule 16, she is exonerated under rule 21), she hasn't a feather to fly with against rule 13.

Even if I were in my rights to delay the tack, I think I would be hesitant to put one in with a boat close behind, unless I felt 100% confident to pull the tack off and clear him.

Standard protection is to luff to head to wind alongside the mark, and make the trailing boat overlapped outside you. Once overlapped, mark room includes room to tack. 

Harder in current/tide though. 

Two boats on opposite tacks approach the windward mark which they are rounding to port. Boat P, on port, tacks below boat S which is approaching on starboard. Boat P's tack is clean and boat S does not have to take any avoiding action. Boat P who cannot lay the mark without luffing shoots the mark and rounds cleanly but Boat S's boom touched Boat P's rigging when this manoeuvre is carried out. 

As this took place in the Zone my view would that it would be unlikely that P could give S the required room to keep clear in all but the lightest of conditions (rule 15).

One way of looking at the 'prevent' part of rule 18.2( a ) is to consider it the 'compliment' of the 'cause to sail above close hauled' part:  where the fetching boat needs to sail above close hauled to avoid contact, but doesn't, isn't that where 'prevents' kicks in?

Are we assuming B is effectively on the lay line though as the illustrations show B to be above it (this is why judging a layline accurately is important)?

If Y had not tacked below B but had been abreast and on the same starboard tack and on the layline within 2 boat lengths and the wind shifted 15 deg.(header) would Y not be entitled to shoot the mark with B giving room? 

You cannot assume anything. Whether the wind shifted or not would be a matter for the PC to determine in their 'facts found' as would whether or not B was (far enough) above the layline or not.

Breaking it down. Rule 18 'switches on' when Y reaches the zone at point 1. As they are on opposite tacks at this point Rule 18 does not apply (18.1a).

When Y tacks at point 3 they are not entitled to 'Mark Room' as the rules clearly state that 18.2 does not apply between them.

18.3a allows Y to sail above close hauled as long as they do not cause B to sail above close hauled or prevent them from rounding the mark. As there was contact between them it shows that something clearly went wrong. 

As Y also changed course to go above I believe Rule 15 applies.

So unless Y can prove that B was well above the layline and could have changed course to a close hauled course then the odds are definitely not in their favour but B is also not required to react to Y unless it is obvious they are not keeping clear. B may have assume Y would sail astern of her and tack.

Without a witness the case for Y does not look good. My gut feeling in this case is that most PCs would find in favour of B unless there is a witness involved.

The rules are stacked against latecomers at a windward mark on port, for a good reason IMO. 

So my advice is, if you are going to come in late on port make sure you have a gap to go in and if it doesn;t work you need to bail out (bail out protest later).

That's certainly the example the commentaries discuss with respect to 'prevent'.
 

Provided boats had been overlapped, not clear ahead/astern when the first of them reached three boatlengths from the mark

Absobleedinglutely.

I would guess it is Rule 11 as Y is not preventing B from passing the mark on the correct side (so doesn't break 18.3) as B is well above the layline they could head up..

How is that not 'preventing' B from passing the mark?

Can B pass the mark without hitting Y?

Isn't that Y preventing B from passing the mark?

We are discussing the operation of a rule.  Of course the example is contrived.  I'm trying to examine what happens at the boundaries of the rule.

I disagree that it couldn't happen in practice.

If Y had passed head to wind before reaching the zone, it would be a routine mark-trap, so all that is needed is for Y to misjudge her position with respect to the zone boundary when she tacks.

B has never been on the layline.  B is always well above the layline.

If B is on the layline, she's nearly always going to get the benefit of rule 18.3( a ) first sentence.

18.3 states:

(a) shall not cause the other boat to sail above close-hauled to avoid contact or prevent the other boat from passing the mark on the required side,

In your contrived example B is well above the layline and could come up to a close hauled course and still leave the mark on the required side. Once they are past the mark it becomes a windward leeward situation.

There is no mention of B being entitled to sail their proper course in 18.3 just that they be allowed to leave the mark on the required side. Which, in this case, had they come up to a close hauled course, they could have.

This is why being on the layline is key as it closes off these situations.