Chris,

You posted this on the Match and Team Racing Rules forum, and since team racing uses the regular rule 18.3, which doesn't have parts (a) and (b), I assume you're asking about the match-race rule, and the rounding is to starboard. When you say, "B is passing the windward mark," I assume that means passing it on the required side. I also assume that B entered the zone either overlapped inside Y or clear ahead, so she is entitled to mark-room, i.e., room to sail her proper course at the mark.

As you say, B may luff Y; you state, "may luff Y head to wind" but in fact if Y must eventually tack to keep clear, she has to do so.

Your second question comes down to the question of what is a proper course. The answer is not clear, but I think umpires would need a geometrical argument for saying that B's proper course is to gybe -- for example, that the next mark is well to the right, looking downwind, so she must gybe to go to it. Call N5 comes close to telling us that it's the boat herself that determines proper course, within geometrical constraints. It says, " [Blue] has a choice of proper courses and may therefore choose one that will maximise her interference with Yellow. Sailing on either tack can be a proper course." Note that there is no mention of there being more pressure on the left, or whatever -- B can simply decide which tack is her proper course.

There is other support in the Call Book for this conclusion. In Call E1, Y gybes at the mark, and even though there is no indication that the right side (looking downwind) is favored, the Call asserts that it is her proper course to gybe at position 5. Without further information it is clearly her proper course to sail on without gybing, so the inference is the same as in Call N5: the boat herself can decide which tack is her proper course. In Call E6, Y's proper course is clearly to tack -- this is what I mean by a "geometrical argument"; the geometry is such that her proper course is obvious. The same is true of Calls J1 and J2. Call K4 shows a diagram in which it seems clear B is not sailing her proper course, yet leaves it to the umpires to decide whether this is her proper course or not. In Call K5, Q1 posits that the umpires have already decided B is sailing her proper course, so the decision appears to be up to them. (Q2/A2 of the same Call mysteriously claims that although the definition "mark-room" only allows B room to sail her proper course and she has no proper course, she is still entitled to room to gybe in front of Y; but in any case, there's no judgement required about whether she's sailing her proper course.) Call N9 asserts that a boat beating to windward that luffs above close-hauled is not sailing her proper course, but that makes sense, as sailing above close-hauled for any period of time is slow. Note that, again, there is no reference to the conditions; you and I know that it might well be Y's proper course to luff -- for example, maybe a 30-knot gust just hit her and she's feathering up to keep the boat up -- but the Call ignores such considerations.

So my call on your second question is going to be that B can decide which tack is her proper course. She need not gybe at the mark.

One final comment: You say, "so long as one boat is still in the zone ..." I don't think rule 18 continues to apply that long. MR Call E2 says, in effect, that rule 18 turns off when the boat entitled to mark-room is sailing on the next leg and the mark is clear astern of her. I grant you that this writes a rule, but as a Match Race Call it's authoritative in match racing.